Add entity at index, TiledLevel changes

* Entities can be added to an EntityContainer (Level, Layer) at a given
index, instead of always at the end of the list.
* The size of a TiledLevel can be set without having to load an image.
* TiledLevel.renderTile() is now called for every tile, whether or not
there exists a tile code that was defined (if it wasn't, null is passed
in as the Tile). It is not called for tiles that are out of bounds
though.