Store Targetables in Actor.

This can be used to avoid several lookups for these traits, as well as allow Actor to provide specialised methods to deal with target types efficiently. This also reduces some code duplication.

OpenRA.Game/Traits/Player/FrozenActorLayer.cs

@@ -59,7 +59,7 @@ namespace OpenRA.Traits
 
 			CenterPosition = self.CenterPosition;
 			Bounds = self.Bounds;
-			TargetTypes = self.TraitsImplementing<ITargetable>().Where(Exts.IsTraitEnabled).SelectMany(t => t.TargetTypes).ToHashSet();
+			TargetTypes = self.GetEnabledTargetTypes().ToHashSet();
 
 			UpdateVisibility();
 		}