v3.8.0: OpenTracks

Breaking change:
- GPX import/export speed, cadence, and heartrate via TrackPointExtension/v2

Changes:
- Dashboard API: provide access to markers (incl. callback to show in OpenTracks)
- New marker icon (orange push pin, SVG)
- Files to import can be selected via file manager

Bugfixes:
- Aggregated stats UI respects preferred rate
- Editing a marker failed on orientation change

Developer:
- Refactored TrackRecordingService
