v3.15.0: OpenTracks

WARNING: 
This release applies database changes that CANNOT BE REVERTED.
As KMZ/KML export/import behaviour changes as well, it is recommended to (A) take a backup BEFORE upgrading and (B) afterwards.
Recommended format: KMZ.

BREAKING CHANGES:
- Internal storage of track segments is changed
  (user generated events are now considered trackpoints without a location but with timestamp)
- Device time instead of GPS time is stored 
  (existing data is not modified)
- Dashboard