Current location tracking

[karussell]

This should properly fix #282 including the 'tracking'.

Test https://graphhopper.com/maps-dev/current_location_tracking/

Input from #430 (thanks @oblonski!) and #279.

This is how it works (should be self-explanatory)

1. After location button is pressed the current location is searched (gray location button pulses)
2. Once found the location button appears blue and the map view is synchronized to the current location
3. If the view is changes the location button looses its blue dot. Pressing it again synchs view again.
4. If the view is synched and location button is pressed again then location fetching is stopped.
5. If there is an error or no permission is granted or no location is enabled for the device, then a red question mark appears in the location button