Centralise and harden editor-ready-for-use check

Not only does this combine the check into one location, but it also adds
a check on the global `WorkingBeatmap` being updated, which is the only
way I can see the following failure happening:

```csharp
05:19:07     osu.Game.Tests: osu.Game.Tests.Visual.Editing.TestSceneEditorBeatmapCreation.TestAddAudioTrack
05:19:07       Failed TestAddAudioTrack [161 ms]
05:19:07       Error Message:
05:19:07        TearDown : System.NullReferenceException : Object reference not set to an instance of an object.
05:19:07       Stack Trace:
05:19:07       --TearDown
05:19:07        at osu.Game.Database.ModelManager`1.<>c__DisplayClass7_0.<AddFile>b__0(TModel managed) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 36
05:19:07        at osu.Game.Database.ModelManager`1.<>c__DisplayClass8_0.<performFileOperation>b__0(Realm realm) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 49
05:19:07        at osu.Game.Database.RealmExtensions.Write(Realm realm, Action`1 function) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\RealmExtensions.cs:line 14
05:19:07        at osu.Game.Database.ModelManager`1.performFileOperation(TModel item, Action`1 operation) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 46
05:19:07        at osu.Game.Database.ModelManager`1.AddFile(TModel item, Stream contents, String filename) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 36
05:19:07        at osu.Game.Screens.Edit.Setup.ResourcesSection.ChangeAudioTrack(FileInfo source) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Screens\Edit\Setup\ResourcesSection.cs:line 115
05:19:07        at osu.Game.Tests.Visual.Editing.TestSceneEditorBeatmapCreation.<TestAddAudioTrack>b__13_0() in C:\BuildAgent\work\ecd860037212ac52\osu.Game.Tests\Visual\Editing\TestSceneEditorBeatmapCreation.cs:line 101
05:19:07        at osu.Framework.Testing.Drawables.Steps.AssertButton.checkAssert()
05:19:07        at osu.Framework.Testing.Drawables.Steps.StepButton.PerformStep(Boolean userTriggered)
05:19:07        at osu.Framework.Testing.TestScene.runNextStep(Action onCompletion, Action`1 onError, Func`2 stopCondition)
```

osu.Game.Rulesets.Osu.Tests/Editor/TestSceneSliderVelocityAdjust.cs

@@ -10,7 +10,6 @@ using osu.Framework.Input;
 using osu.Framework.Testing;
 using osu.Framework.Utils;
 using osu.Game.Beatmaps;
-using osu.Game.Rulesets.Edit;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Rulesets.UI;
 using osu.Game.Screens.Edit;
@@ -41,10 +40,6 @@ namespace osu.Game.Rulesets.Osu.Tests.Editor
 
         protected override IBeatmap CreateBeatmap(RulesetInfo ruleset) => new TestBeatmap(ruleset, false);
 
-        private bool editorComponentsReady => editor.ChildrenOfType<HitObjectComposer>().FirstOrDefault()?.IsLoaded == true
-                                              && editor.ChildrenOfType<TimelineArea>().FirstOrDefault()?.IsLoaded == true
-                                              && editor?.ChildrenOfType<Playfield>().FirstOrDefault()?.IsLoaded == true;
-
         [TestCase(true)]
         [TestCase(false)]
         public void TestVelocityChangeSavesCorrectly(bool adjustVelocity)
@@ -52,7 +47,7 @@ namespace osu.Game.Rulesets.Osu.Tests.Editor
             double? velocity = null;
 
             AddStep("enter editor", () => Game.ScreenStack.Push(new EditorLoader()));
-            AddUntilStep("wait for editor load", () => editorComponentsReady);
+            AddUntilStep("wait for editor load", () => editor?.ReadyForUse == true);
 
             AddStep("seek to first control point", () => editorClock.Seek(editorBeatmap.ControlPointInfo.TimingPoints.First().Time));
             AddStep("enter slider placement mode", () => InputManager.Key(Key.Number3));
@@ -91,7 +86,7 @@ namespace osu.Game.Rulesets.Osu.Tests.Editor
             AddStep("exit", () => InputManager.Key(Key.Escape));
 
             AddStep("enter editor (again)", () => Game.ScreenStack.Push(new EditorLoader()));
-            AddUntilStep("wait for editor load", () => editorComponentsReady);
+            AddUntilStep("wait for editor load", () => editor?.ReadyForUse == true);
 
             AddStep("seek to slider", () => editorClock.Seek(slider.StartTime));
             AddAssert("slider has correct velocity", () => slider.Velocity == velocity);

osu.Game.Tests/Visual/Editing/TestSceneEditorBeatmapCreation.cs

@@ -36,8 +36,6 @@ namespace osu.Game.Tests.Visual.Editing
     {
         protected override Ruleset CreateEditorRuleset() => new OsuRuleset();
 
-        protected override bool EditorComponentsReady => Editor.ChildrenOfType<SetupScreen>().SingleOrDefault()?.IsLoaded == true;
-
         protected override bool IsolateSavingFromDatabase => false;
 
         [Resolved]

osu.Game.Tests/Visual/Editing/TestSceneEditorNavigation.cs

@@ -9,12 +9,10 @@ using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Testing;
 using osu.Game.Beatmaps;
 using osu.Game.Database;
-using osu.Game.Rulesets.Edit;
 using osu.Game.Rulesets.Mania;
 using osu.Game.Rulesets.Osu;
 using osu.Game.Screens.Edit;
 using osu.Game.Screens.Edit.Components.Timelines.Summary;
-using osu.Game.Screens.Edit.Compose.Components.Timeline;
 using osu.Game.Screens.Edit.GameplayTest;
 using osu.Game.Screens.Select;
 using osu.Game.Tests.Resources;
@@ -40,10 +38,7 @@ namespace osu.Game.Tests.Visual.Editing
             AddStep("switch ruleset", () => Game.Ruleset.Value = new ManiaRuleset().RulesetInfo);
 
             AddStep("open editor", () => ((PlaySongSelect)Game.ScreenStack.CurrentScreen).Edit(beatmapSet.Beatmaps.First(beatmap => beatmap.Ruleset.OnlineID == 0)));
-            AddUntilStep("wait for editor open", () => Game.ScreenStack.CurrentScreen is Editor editor
+            AddUntilStep("wait for editor open", () => Game.ScreenStack.CurrentScreen is Editor editor && editor.ReadyForUse);
-                                                       && editor.IsLoaded
-                                                       && editor.ChildrenOfType<HitObjectComposer>().FirstOrDefault()?.IsLoaded == true
-                                                       && editor.ChildrenOfType<TimelineArea>().FirstOrDefault()?.IsLoaded == true);
             AddStep("test gameplay", () =>
             {
                 var testGameplayButton = this.ChildrenOfType<TestGameplayButton>().Single();

osu.Game/Screens/Edit/Editor.cs

@@ -21,6 +21,7 @@ using osu.Framework.Input.Events;
 using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Framework.Screens;
+using osu.Framework.Testing;
 using osu.Framework.Timing;
 using osu.Game.Audio;
 using osu.Game.Beatmaps;
@@ -39,6 +40,7 @@ using osu.Game.Rulesets.Edit;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Screens.Edit.Components.Menus;
 using osu.Game.Screens.Edit.Compose;
+using osu.Game.Screens.Edit.Compose.Components.Timeline;
 using osu.Game.Screens.Edit.Design;
 using osu.Game.Screens.Edit.GameplayTest;
 using osu.Game.Screens.Edit.Setup;
@@ -97,6 +99,32 @@ namespace osu.Game.Screens.Edit
 
         public IBindable<bool> SamplePlaybackDisabled => samplePlaybackDisabled;
 
+        /// <summary>
+        /// Ensure all asynchronously loading pieces of the editor are in a good state.
+        /// This exists here for convenience for tests, not for actual use.
+        /// Eventually we'd probably want a better way to signal this.
+        /// </summary>
+        public bool ReadyForUse
+        {
+            get
+            {
+                if (!workingBeatmapUpdated)
+                    return false;
+
+                var loadedScreen = screenContainer?.Children.SingleOrDefault(s => s.IsLoaded);
+
+                if (loadedScreen == null)
+                    return false;
+
+                if (loadedScreen is EditorScreenWithTimeline)
+                    return loadedScreen.ChildrenOfType<TimelineArea>().FirstOrDefault()?.IsLoaded == true;
+
+                return true;
+            }
+        }
+
+        private bool workingBeatmapUpdated;
+
         private readonly Bindable<bool> samplePlaybackDisabled = new Bindable<bool>();
 
         private bool canSave;
@@ -219,6 +247,7 @@ namespace osu.Game.Screens.Edit
                 // this assumes that nothing during the rest of this load() method is accessing Beatmap.Value (loadableBeatmap should be preferred).
                 // generally this is quite safe, as the actual load of editor content comes after menuBar.Mode.ValueChanged is fired in its own LoadComplete.
                 Beatmap.Value = loadableBeatmap;
+                workingBeatmapUpdated = true;
             });
 
             OsuMenuItem undoMenuItem;

osu.Game/Tests/Visual/EditorTestScene.cs

@@ -17,9 +17,7 @@ using osu.Game.Online.API;
 using osu.Game.Overlays;
 using osu.Game.Overlays.Dialog;
 using osu.Game.Rulesets;
-using osu.Game.Rulesets.Edit;
 using osu.Game.Screens.Edit;
-using osu.Game.Screens.Edit.Compose.Components.Timeline;
 using osu.Game.Screens.Menu;
 using osu.Game.Skinning;
 
@@ -58,15 +56,13 @@ namespace osu.Game.Tests.Visual
                 Dependencies.CacheAs<BeatmapManager>(testBeatmapManager = new TestBeatmapManager(LocalStorage, Realm, rulesets, null, audio, Resources, host, Beatmap.Default));
         }
 
-        protected virtual bool EditorComponentsReady => Editor.ChildrenOfType<HitObjectComposer>().FirstOrDefault()?.IsLoaded == true
-                                                        && Editor.ChildrenOfType<TimelineArea>().FirstOrDefault()?.IsLoaded == true;
-
         public override void SetUpSteps()
         {
             base.SetUpSteps();
 
             AddStep("load editor", LoadEditor);
-            AddUntilStep("wait for editor to load", () => EditorComponentsReady);
+            AddUntilStep("wait for editor to load", () => Editor?.ReadyForUse == true);
+            AddUntilStep("wait for beatmap updated", () => !Beatmap.IsDefault);
         }
 
         protected virtual void LoadEditor()