v4.24.0: OpenTracks

Changes:
- PublicAPI: start track only sets field if data is present
- Marker photos can be shared again
- Removed KMZ without photos
- Heart rate of 0.0 is treated as invalid (except for SensorStatistics)
- Removed import workaround for special coordinates (100/200)
  (was used by MyTracks to export/import special trackpoints)

Developer:
- Track and Marker are records
- AggregatedStatistics are computed in database
- Split into TrackStatisticsUpdater and SegmentStatisticUpdater
- Introduced Iterator classes (instead of passing Cursor or List)