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.Mods.Common/Warheads/Warhead.cs

@@ -56,8 +56,7 @@ namespace OpenRA.Mods.Common.Warheads
 				return false;
 
 			// A target type is valid if it is in the valid targets list, and not in the invalid targets list.
-			var targetable = victim.TraitsImplementing<ITargetable>().Where(Exts.IsTraitEnabled);
-			if (!IsValidTarget(targetable.SelectMany(t => t.TargetTypes)))
+			if (!IsValidTarget(victim.GetEnabledTargetTypes()))
 				return false;
 
 			return true;