Introduce IExpandable interface

osu.Game/Overlays/IExpandable.cs

@@ -0,0 +1,19 @@
+// 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.
+
+using osu.Framework.Bindables;
+using osu.Framework.Graphics;
+
+namespace osu.Game.Overlays
+{
+    /// <summary>
+    /// An interface for drawables with ability to expand/contract.
+    /// </summary>
+    public interface IExpandable : IDrawable
+    {
+        /// <summary>
+        /// Whether this drawable is in an expanded state.
+        /// </summary>
+        BindableBool Expanded { get; }
+    }
+}