Standardise drawable state access and split large nested classes out of MainMenu.ButtonSystem

osu.Game/GameModes/Menu/FlowContainerWithOrigin.cs

@@ -0,0 +1,36 @@
+using OpenTK;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+
+namespace osu.Game.GameModes.Menu
+{
+    /// <summary>
+    /// A flow container with an origin based on one of its contained drawables.
+    /// </summary>
+    public class FlowContainerWithOrigin : FlowContainer
+    {
+        /// <summary>
+        /// A target drawable which this flowcontainer should be centered around.
+        /// This target MUST be in this FlowContainer's *direct* children.
+        /// </summary>
+        internal Drawable CentreTarget;
+
+        public override Anchor Origin => Anchor.Custom;
+
+        public override Vector2 OriginPosition
+        {
+            get
+            {
+                if (CentreTarget == null)
+                    return base.OriginPosition;
+
+                return CentreTarget.Position + CentreTarget.Size / 2;
+            }
+        }
+
+        public FlowContainerWithOrigin()
+        {
+            Direction = FlowDirection.HorizontalOnly;
+        }
+    }
+}