Renamed TmxTileMap to SfmlTileMap.
Slightly restructured SfmlTileMap class.

TmxTileMap is passed in on loading rather than being maintained in the
background.
Completely separated SfmlTileMap from TmxTileMap.

SfmlTileMap now works independently of TmxTileMap, so that any changes to the
latter (including deletion) do not affect the former.