- Added comments.
- Now the tileset struct keeps a vector of collision rects for each tile.
- The tileset loading method expects, for each XML collision rect node, a attribute "tiles" specifying one of the following options:
	- "all" : the collision rect is added to all the tiles in the tileset.
	- <first>-<last> : the collision rect is added to all the tiles in the range [first, last].
	- <i> : the collision rect is added to the tile i.