Refactor skin configuration to be infinitely extensible

osu.Game/Skinning/GlobalSkinConfiguration.cs

@@ -0,0 +1,10 @@
+// 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.Skinning
+{
+    public enum GlobalSkinConfiguration
+    {
+        ComboColours
+    }
+}