Redesign app and remove the MainActivity

[boldtrn]

This PR introduces a minor redesign, removes the main activity, adds the correct launcher icon, and a couple of other minor things.

I am not sure if the current design is a bit too extreme. I used the GraphHopper green, but it might be a bit too much and we might want something less colorful? If you prefer other colors, just let me know :).

[karussell]

Thanks - good to merge for me :+1: