Manually fire editor save operations on skin change

This is invariably a test fix but also a valid change to make in
isolation.

In short, the problem is thus: Now that the beatmap skin is hooked up to
realm to receive notifications about changed files (which is required
for correctly handling custom samples), tests started failing, because
of the following sequence of events

- Saving a brand new `.osu` to a beatmap set causes
  `RealmBackedResourceStore` to fire subscription callbacks because a
  new file was added to the set
- This fires `RealmBackedResourceStore.CacheInvalidated`
- Which fires `EditorBeatmapSkin.BeatmapSkinChanged`
- Which would previously fire `EditorBeatmap.SaveState()` and as such
  mark the beatmap dirty / modified.

In this scenario this is gratuitous. There's no need to be raising save
states here, a new `.osu` was added to the set that is in a consistent
saved state and nothing actually changed in the beatmap.

However it does not appear sane to attempt to circumvent this with
conditional guards or something, because in cases where files are
added/removed from the set, *there isn't really any reason to take save
states anyway*. The change handler only deals with the `.osu`, any
modifications to any of the other files cannot be undone anyway.

Therefore, only keep the state save to the one change to beatmap skin
that *can* actually be sanely undone which is changing combo colours.

osu.Game/Screens/Edit/EditorBeatmap.cs

@@ -102,10 +102,7 @@ namespace osu.Game.Screens.Edit
             this.beatmapInfo = beatmapInfo ?? playableBeatmap.BeatmapInfo;
 
             if (beatmapSkin is LegacyBeatmapSkin skin)
-            {
-                BeatmapSkin = new EditorBeatmapSkin(playableBeatmap.BeatmapInfo!.BeatmapSet!, skin);
-                BeatmapSkin.BeatmapSkinChanged += SaveState;
-            }
+                BeatmapSkin = new EditorBeatmapSkin(this, skin);
 
             beatmapProcessor = new EditorBeatmapProcessor(this, playableBeatmap.BeatmapInfo.Ruleset.CreateInstance());
 

osu.Game/Screens/Edit/EditorBeatmapSkin.cs

@@ -10,7 +10,6 @@ using osu.Framework.Bindables;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Textures;
 using osu.Game.Audio;
-using osu.Game.Beatmaps;
 using osu.Game.Skinning;
 using osuTK.Graphics;
 
@@ -34,10 +33,13 @@ namespace osu.Game.Screens.Edit
         /// </summary>
         public BindableList<Colour4> ComboColours { get; }
 
-        public EditorBeatmapSkin(BeatmapSetInfo beatmapSet, LegacyBeatmapSkin skin)
-        {
-            Skin = skin;
+        private readonly EditorBeatmap editorBeatmap;
 
+        public EditorBeatmapSkin(EditorBeatmap editorBeatmap, LegacyBeatmapSkin skin)
+        {
+            this.editorBeatmap = editorBeatmap;
+
+            Skin = skin;
             ComboColours = new BindableList<Colour4>();
 
             if (Skin.Configuration.ComboColours is IReadOnlyList<Color4> comboColours)
@@ -67,6 +69,7 @@ namespace osu.Game.Screens.Edit
             for (int i = 0; i < ComboColours.Count; ++i)
                 Skin.Configuration.CustomComboColours.Add(ComboColours[(ComboColours.Count + i - 1) % ComboColours.Count]);
             InvokeSkinChanged();
+            editorBeatmap.SaveState();
         }
 
         #endregion