Many large changes which should really be split up

- Fix consistency checks
- Factor out methods
- Improve display
Move notes to README.md
Use ':' as a separator rather than '/'
Factor out Lap.isEmpty()
Add end and delta time to Lap display
Add start, end and delta distance to Lap display
Factor out Activity.isEmpty()
Add start, end and delta time to Activity display
Factor out some helpers used by the app and by tests
Add tests
Fix removing time-only tracks when merging laps