AnimatedTile support in TmxMapLoader

AnimatedTiledMapTile doesn't support variable animation intervals, while the TMX format does. As a result, I set the interval in the frame loading loop, but only the last value gets used.  Good enough for me.  We can extend the AnimatedTiledMapTile in the future if we choose, and update the loader accordingly.

The TMX format is backwards-compatible with clients that don't support animations.  To do this, animated tiles replace the ids of static tiles (which may be used in the animation) so that Old clients will "see" a static tile. For example:

Tile 1 - Static tile
Tile 1 - Static tile
Tile 1 - Static tile
Tile 1 - Static tile

Thanks Tomski!