store traits differently (index on trait class rather than actor)

OpenRA.Game/World.cs

@@ -25,6 +25,7 @@ namespace OpenRA
 {
 	public class World
 	{
+		internal TraitDictionary traitDict = new TraitDictionary();
 		Set<Actor> actors = new Set<Actor>();
 		List<IEffect> effects = new List<IEffect>();
 		Queue<Action<World>> frameEndActions = new Queue<Action<World>>();