Make control group hotkeys configurable

- Split control groups management to its own interface
- Add hotkeys for selecting, creating, adding to and combining with control groups
- Add a ControlGroups widget to manage the player interaction

OpenRA.Mods.Common/Widgets/Logic/Ingame/Hotkeys/RemoveFromControlGroupHotkeyLogic.cs

@@ -38,7 +38,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic.Ingame
 				.ToArray();
 
 			foreach (var a in selectedActors)
-				selection.RemoveFromControlGroup(a);
+				world.ControlGroups.RemoveFromControlGroup(a);
 
 			return true;
 		}