v4.26.0: OpenTracks

Changes:
- GPS is optional: configurable in the settings
- BLE sensor address/name is only shown after successful connection

Bugfix:
- Altitude is now shown again in EGM2008 (introduced in v4.25.0)
- Altitude and speed were shown as 0.0 briefly when data was stored

Developer:
- Refactored sensor subsystem