smaller improvements

TS has seen a few smaller improvements.
* the isometric view follows the car and is nearly complete (turning it is missing)
* the download-menu is loaded asynchronously to speed up the application-start if there is no network
* the CenterOnGPS-button may no longer throw an exception if we have GPS but no location-fix yet.
* we are currently trying to improve the time to import a map but this takes a while. (it's way too slow at the moment)