disallow adding actors from other players to control groups

closes #4949

CHANGELOG

@@ -165,6 +165,7 @@ NEW:
 		Renamed ChronoshiftDeploy trait to PortableChrono.
 		Added LineBuildNode trait to filter which structure(s) a LineBuild actor can be attached to.
 		Updated to SDL 2.0.3 under OSX, which improves platform integration.
+		Fixed an exploit where you could add control groups on enemy units.
 	Server:
 		Message of the day is now shared between all mods and a default motd.txt gets created in the user directory.
 	Asset Browser:

OpenRA.Game/Selection.cs

@@ -85,7 +85,7 @@ namespace OpenRA
 				for (var i = 0; i < 10; i++)	/* all control groups */
 					controlGroups[i].RemoveAll(a => actors.Contains(a));
 
-				controlGroups[group].AddRange(actors);
+				controlGroups[group].AddRange(actors.Where(a => a.Owner == world.LocalPlayer));
 				return;
 			}