Project feedback

Note: based on PR #1523

TODO (all enhancements):
- general "entering a failsafe mode" tone
- failsafe LAND warning siren
- rework arm/disarm gesture to use a timer instead of a counter
- implement lost copter beacon when ((crashed or failsafe landed) and radio failsafe) or (when user holds stick in disarm position)
- make sure a battery failsafe will continue to play after a lost copter beacon stops