This class iterates recursively over layers, flattening groups.
This enables us to simplify the code when we iterate layers. We can remove all recursive function calls in the GameScene code (it is delegated to the LayersIterator)
This commit adds a listener in the preload function that will be triggered as soon as the map is loaded.
This function will load the resources from the map (tilesets) defined in the map.
That way, we don't have to define manually the list of tiles that have to be loaded (at the expense of a slight delay in loading since we must wait for the map to be loaded to start loading the tiles).
