BMapPlayfield save state

Currently, BMapPlayfield loads its data as described in the "tilemap".files.txt file. The attributes and tile indexes are combined so the attributes (code, "wall" bits) into TUint32 per position in the map.

This allows the game logic to fiddle with the attribute bits, or modify the map (e.g. playfield animation), while being able to reload the map and have none of those changes made.

In a game like modite-adventure, you may want to enter a dungeon and then exit, and then enter it again. You probably want the dungeon state to be the same as when you left it.

In order to persist the dungeon state, the BMapPlayfield class needs to be able to save its state to persistent storage (e.g. file system in flash), and then be able to load from the persisted data.