Add the ability for PerformFromMenuRunner to inspect nested screen stacks

osu.Game/Screens/IHasSubScreenStack.cs

@@ -0,0 +1,15 @@
+// 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.Screens;
+
+namespace osu.Game.Screens
+{
+    /// <summary>
+    /// A screen which manages a nested stack of screens within itself.
+    /// </summary>
+    public interface IHasSubScreenStack
+    {
+        ScreenStack SubScreenStack { get; }
+    }
+}