Add simplified implementation of an expandable slider

osu.Game/Overlays/IExpandableControl.cs

@@ -0,0 +1,16 @@
+// 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.Overlays
+{
+    /// <summary>
+    /// An interface for UI controls with the ability to expand/contract.
+    /// </summary>
+    public interface IExpandableControl : IExpandable
+    {
+        /// <summary>
+        /// Returns whether the UI control is currently in a dragged state.
+        /// </summary>
+        bool IsControlDragged { get; }
+    }
+}