From now on, GameMap is the central container for objects
and is acting as a scene was before.
What's more, GameMap is not restricted to 4 layers anymore.
User can combine TileLayers (which define the terrain)
and ObjectLayers (which can contain movable objects) together
and is no limited to have only one ObjectLayer.
Thanks to that it would be easier to implement i.e. birds which actually
are a movable objects and are flying above the map and the highest
terrain (top TileLayers in fact).