Read fewer fields from TMX files

* Do not read "tilewidth" and "tileheight" from <map>, always assume 32.
* Do not read "tilewidth" and "tileheight" from <tileset>, always assume 32.
* Do not read "width" and "height" from <layer>, always assume same size as map.
* Do not read "width" and "height" from <objectgroup>, always assume same size as map.
* Do not read <image> inside <tileset>. Always assume tileset name is same as image resource name from ResourceLoader.