v3.9.0: OpenTracks

Changes:
- Export tracks with their UUID
- Prevent import of already existing tracks by UUID (configurable; default yes)
- Pause/stop button now require long press
- GPS pre-start shows status of fix
- Added new activity icons
- Speed sensor: wheelsize now configurable in millimeter

Bugfixes:
- Fixed some rare crashes (NPE)
- Pace formatting corrected
- App icon in launcher repositioned
