addNewEntity(world, List<Component> cs, {String player, List<String> groups}) {
 var e = world.createEntity();
 cs.forEach((c) => e.addComponent(c));
 if (groups != null) {
   var gm = (world.getManager(GroupManager) as GroupManager);
   groups.forEach((group) => gm.add(e, group));
 }
 if (player != null) {
   (world.getManager(PlayerManager) as PlayerManager).setPlayer(e, player);
 }
 world.addEntity(e);
 return e;
}