LevelChunk class created

Created class LevelChunk which is a special type of Node.
LevelChunk integration, lights on chunks

Changed the LevelControl so that it uses the LevelChunk class. Added a
lightsource to each chunk. In order for the light to illuminate the
player, the LevelControl has to be attached directly to the gameNode,
not the lower levelNode as before (this change is in
InGameState.initLevel() ).