Replace DifficultyColouredContainer with a more scalable solution

osu.Game/Beatmaps/DifficultyRating.cs

@@ -0,0 +1,15 @@
+// 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.
+
+namespace osu.Game.Beatmaps
+{
+    public enum DifficultyRating
+    {
+        Easy,
+        Normal,
+        Hard,
+        Insane,
+        Expert,
+        ExpertPlus
+    }
+}