Created an FGTileMgr class to encapsulate the high level tile management

Created an FGTileMgr class to encapsulate the high level tile management
  tasks.
Created a tile load queue.  When we init, or cross a tile boundary, we
  stuff the new tiles to be loaded on a load queue, and then only load
  one tile per frame.  This will need further refinement, but it is better
  than what we had.