Handle Tile level object groups #76

Tiled allows users to edit tilesets so that each tile has an individual collision object. When the tile is drawn to the screen, the object is drawn at the same time.

The object is drawn to a hidden (from Tiled) object-group layer, that exists within the Tile tag.

Need to decode these, and add them to the object "getting stack", so they can be retrieved from a map level.