Add skin source fallback chain

osu.Game/Skinning/ISkinSource.cs

@@ -0,0 +1,25 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using osu.Framework.Audio.Sample;
+using osu.Framework.Graphics;
+using osu.Game.Rulesets.Objects.Types;
+using OpenTK.Graphics;
+
+namespace osu.Game.Skinning
+{
+    /// <summary>
+    /// Provides access to skinnable elements.
+    /// </summary>
+    public interface ISkinSource
+    {
+        event Action SourceChanged;
+
+        Drawable GetDrawableComponent(string componentName);
+
+        SampleChannel GetSample(string sampleName);
+
+        Color4? GetComboColour(IHasComboIndex comboObject);
+    }
+}