Big refactoring of ABC / arcade level : part 1, loading tmx maps in ABC

This is part of the big refactoring to move most of "graphics" stuff into the ABC level class. Actually, what was (and still is) in level_arcade is not really arcade dependent, it is mainly because this is code intended to communicate with the arcade GUI, but actually it is just sending payloads in the output_queue. So we can also use it in test mode, by connecting the testing library to the output_queue so we can check the payloads if we want to.