Stash TMX changes.
Better support for XML resources, and an API of helpers on XMLResource class.
 - Load tiled up to the tileset.  Need to load layers and objectgroups next.
 - Support dependency loading before root resource is complete.
Convert tile maps over to TMX for maps and TSX for tilesets.  These are XML formats that are created by Tiled Map Editor.
 - Add support for editing maps in Tiled.
Merge branch 'jdd-tmx' into develop