Move available mods to global context

This also tidies up ModSelectOverlay and setting creation flow in general.

osu.Game/OsuGameBase.cs

@@ -83,6 +83,11 @@ namespace osu.Game
         [Cached(typeof(IBindable<IReadOnlyList<Mod>>))]
         protected readonly Bindable<IReadOnlyList<Mod>> Mods = new Bindable<IReadOnlyList<Mod>>(Array.Empty<Mod>());
 
+        /// <summary>
+        /// Mods available for the current <see cref="Ruleset"/>.
+        /// </summary>
+        public readonly Bindable<Dictionary<ModType, IReadOnlyList<Mod>>> AvailableMods = new Bindable<Dictionary<ModType, IReadOnlyList<Mod>>>();
+
         protected Bindable<WorkingBeatmap> Beatmap { get; private set; } // cached via load() method
 
         private Bindable<bool> fpsDisplayVisible;
@@ -233,6 +238,18 @@ namespace osu.Game
             PreviewTrackManager previewTrackManager;
             dependencies.Cache(previewTrackManager = new PreviewTrackManager());
             Add(previewTrackManager);
+
+            Ruleset.BindValueChanged(onRulesetChanged);
+        }
+
+        private void onRulesetChanged(ValueChangedEvent<RulesetInfo> r)
+        {
+            var dict = new Dictionary<ModType, IReadOnlyList<Mod>>();
+
+            foreach (ModType type in Enum.GetValues(typeof(ModType))) dict[type] = r.NewValue?.CreateInstance().GetModsFor(type).ToList();
+
+            SelectedMods.Value = Array.Empty<Mod>();
+            AvailableMods.Value = dict;
         }
 
         protected virtual Container CreateScalingContainer() => new DrawSizePreservingFillContainer();