Allow skin editor to be invoked from any context

This is kind of how I see things working going forward, where the editor
can be applied to anything in the game which supports it (ie. a results
screen, gameplay screen, etc.) and it will immediately allow changing
the interface.

This adds a test scene which shows this working with gameplay.

osu.Game.Tests/Visual/Gameplay/TestSceneSkinEditorMultipleSkins.cs

@@ -0,0 +1,61 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Testing;
+using osu.Game.Rulesets;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Osu;
+using osu.Game.Rulesets.Scoring;
+using osu.Game.Screens.Play;
+using osu.Game.Skinning.Editor;
+using osuTK.Input;
+
+namespace osu.Game.Tests.Visual.Gameplay
+{
+    public class TestSceneSkinEditorMultipleSkins : SkinnableTestScene
+    {
+        [SetUpSteps]
+        public void SetUpSteps()
+        {
+            AddStep("create editor overlay", () =>
+            {
+                SetContents(() =>
+                {
+                    var ruleset = new OsuRuleset();
+                    var working = CreateWorkingBeatmap(ruleset.RulesetInfo);
+                    var beatmap = working.GetPlayableBeatmap(ruleset.RulesetInfo);
+
+                    ScoreProcessor scoreProcessor = new ScoreProcessor();
+
+                    var drawableRuleset = ruleset.CreateDrawableRulesetWith(beatmap);
+
+                    var hudOverlay = new HUDOverlay(scoreProcessor, null, drawableRuleset, Array.Empty<Mod>())
+                    {
+                        Anchor = Anchor.Centre,
+                        Origin = Anchor.Centre,
+                    };
+
+                    // Add any key just to display the key counter visually.
+                    hudOverlay.KeyCounter.Add(new KeyCounterKeyboard(Key.Space));
+                    hudOverlay.ComboCounter.Current.Value = 1;
+
+                    return new Container
+                    {
+                        RelativeSizeAxes = Axes.Both,
+                        Children = new Drawable[]
+                        {
+                            drawableRuleset,
+                            hudOverlay,
+                            new SkinEditor(hudOverlay),
+                        }
+                    };
+                });
+            });
+        }
+
+        protected override Ruleset CreateRulesetForSkinProvider() => new OsuRuleset();
+    }
+}