Replace IActorInit with an abstract class.

A shared ValueActorInit<T> is introduced to reduce duplication
in the most common init cases, and an ActorInitActorReference
allow actors to be referenced by map.yaml name.

OpenRA.Mods.Common/Traits/Player/PlaceBuilding.cs

@@ -16,7 +16,7 @@ using OpenRA.Traits;
 namespace OpenRA.Mods.Common.Traits
 {
 	// Allows third party mods to detect whether an actor was created by PlaceBuilding.
-	public class PlaceBuildingInit : IActorInit { }
+	public class PlaceBuildingInit : RuntimeFlagInit { }
 
 	[Desc("Allows the player to execute build orders.", " Attach this to the player actor.")]
 	public class PlaceBuildingInfo : TraitInfo