Add basic structure for skin configurations

osu.Game/Skinning/LegacySkinDecoder.cs

@@ -0,0 +1,15 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Game.Beatmaps.Formats;
+
+namespace osu.Game.Skinning
+{
+    public class LegacySkinDecoder : LegacyDecoder<SkinConfiguration>
+    {
+        public LegacySkinDecoder(int version)
+            : base(version)
+        {
+        }
+    }
+}