move to compose namespace and add license header

osu.Game/Screens/Edit/Compose/SelectionHelper.cs

@@ -1,4 +1,7 @@
-using System.Linq;
+// 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 System.Linq;
 using System.Text;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
@@ -6,7 +9,7 @@ using osu.Framework.Platform;
 using osu.Game.Extensions;
 using osu.Game.Rulesets.Edit;
 
-namespace osu.Game.Screens.Edit
+namespace osu.Game.Screens.Edit.Compose
 {
     public class SelectionHelper : Component
     {