Add Actor.ReplacedByActor to track transformations.

This isn't great conceptually, but has precedent
in the Generation number.

OpenRA.Game/Actor.cs

@@ -48,6 +48,7 @@ namespace OpenRA
 		public Activity CurrentActivity { get; private set; }
 
 		public int Generation;
+		public Actor ReplacedByActor;
 
 		public IEffectiveOwner EffectiveOwner { get; private set; }
 		public IOccupySpace OccupiesSpace { get; private set; }

OpenRA.Mods.Common/Activities/Transform.cs

@@ -132,6 +132,8 @@ namespace OpenRA.Mods.Common.Activities
 				foreach (var nt in self.TraitsImplementing<INotifyTransform>())
 					nt.AfterTransform(a);
 
+				self.ReplacedByActor = a;
+
 				if (selected)
 					w.Selection.Add(w, a);