Add SkinnableSound class

Tidy things up, move logic out of SampleInfo.

osu.Game/Skinning/SkinReloadableDrawable.cs

@@ -0,0 +1,53 @@
+// 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.Framework.Allocation;
+using osu.Framework.Configuration;
+using osu.Framework.Graphics.Containers;
+
+namespace osu.Game.Skinning
+{
+    /// <summary>
+    /// A drawable which has a callback when the skin changes.
+    /// </summary>
+    public abstract class SkinReloadableDrawable : CompositeDrawable
+    {
+        private Bindable<Skin> skin;
+
+        /// <summary>
+        /// Whether fallback to default skin should be allowed if the custom skin is missing this resource.
+        /// </summary>
+        private readonly bool allowDefaultFallback;
+
+        /// <summary>
+        /// Create a new
+        /// </summary>
+        /// <param name="fallback">Whether fallback to default skin should be allowed if the custom skin is missing this resource.</param>
+        protected SkinReloadableDrawable(bool fallback = true)
+        {
+            allowDefaultFallback = fallback;
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(SkinManager skinManager)
+        {
+            skin = skinManager.CurrentSkin.GetBoundCopy();
+            skin.ValueChanged += skin => SkinChanged(skin, allowDefaultFallback || skin.SkinInfo == SkinInfo.Default);
+        }
+
+        protected override void LoadAsyncComplete()
+        {
+            base.LoadAsyncComplete();
+            skin.TriggerChange();
+        }
+
+        /// <summary>
+        /// Called when a change is made to the skin.
+        /// </summary>
+        /// <param name="skin">The new skin.</param>
+        /// <param name="allowFallback">Whether fallback to default skin should be allowed if the custom skin is missing this resource.</param>
+        protected virtual void SkinChanged(Skin skin, bool allowFallback)
+        {
+        }
+    }
+}