Add ISingleInstanceInit interface.

Inits that are logically singletons (e.g. actor
location or owner) should implement this interface
to avoid runtime inconsistencies.

Duplicate instances are rejected at init-time,
allowing simpler queries when they are used.

OpenRA.Mods.Common/Traits/Buildings/Building.cs

@@ -278,7 +278,7 @@ namespace OpenRA.Mods.Common.Traits
 		public Building(ActorInitializer init, BuildingInfo info)
 		{
 			self = init.Self;
-			topLeft = init.GetValue<LocationInit, CPos>(info);
+			topLeft = init.GetValue<LocationInit, CPos>();
 			Info = info;
 			influence = self.World.WorldActor.Trait<BuildingInfluence>();