Add INotifyOwnerChanged interface

OpenRA.Game/Actor.cs

@@ -254,10 +254,15 @@ namespace OpenRA
 		{
 			World.AddFrameEndTask(w =>
 			{
+				var oldOwner = Owner;
+
 				// momentarily remove from world so the ownership queries don't get confused
 				w.Remove(this);
 				Owner = newOwner;
 				w.Add(this);
+
+				foreach (var t in this.TraitsImplementing<INotifyOwnerChanged>())
+					t.OnOwnerChanged(this, oldOwner, newOwner);
 			});
 		}
 	}

OpenRA.Game/Traits/TraitsInterfaces.cs

@@ -63,6 +63,7 @@ namespace OpenRA.Traits
 	public interface INotifyAppliedDamage { void AppliedDamage(Actor self, Actor damaged, AttackInfo e); }
 	public interface INotifyBuildComplete { void BuildingComplete(Actor self); }
 	public interface INotifyProduction { void UnitProduced(Actor self, Actor other, CPos exit); }
+	public interface INotifyOwnerChanged { void OnOwnerChanged(Actor self, Player oldOwner, Player newOwner); }
 	public interface INotifyCapture { void OnCapture(Actor self, Actor captor, Player oldOwner, Player newOwner); }
 	public interface INotifyOtherCaptured { void OnActorCaptured(Actor self, Actor captured, Actor captor, Player oldOwner, Player newOwner); }
 	public interface IAcceptSpy { void OnInfiltrate(Actor self, Actor spy); }