[WIP] Use DungeonLevelMap to handle TiledMap abstraction

TODO

* Unload Tiles dynamically when they are not needed

Generating a definite range of Tile objects in a parser could lead to
performance/memory issues for large maps
[WIP] Handle walkable Tiles for Characters

TODO

* Duplicated behavior in FinishMoveToTile#run and DungeonLevelCharacterActor#current_tile=