Cache TargetablePositions in Actor

To speed up Target.Positions.

OpenRA.Game/Traits/TraitsInterfaces.cs

@@ -387,8 +387,11 @@ namespace OpenRA.Traits
 	public interface ITargetablePositions
 	{
 		IEnumerable<WPos> TargetablePositions(Actor self);
+		bool AlwaysEnabled { get; }
 	}
 
+	public interface IMoveInfo : ITraitInfoInterface { }
+
 	[RequireExplicitImplementation]
 	public interface IGameOver { void GameOver(World world); }