Rename AtlasTiledMapLoader to AtlasTmxMapLoader

Since this loader specifically loads .tmx maps it should be named
accordingly. In LibGDX, a TiledMap is not intrinsically tied to the Tiled
application or its tmx format. This name clarifies that the loader
specifically loads tmx files.
Sync GWT emulation of TmxMapLoader to core changes
Sync AtlasTmxMapLoader with TmxMapLoader changes
Add GWT emulation of AtlasTmxMapLoader