New sounds to improve feedback to pilot

- Adds the concept of "events" to AP_Notify, which are automatically reset after the devices are updated.
- Moves the tones list into the APM codebase to make it easier to work with (no more PX4Firmware pull request necessary to change tones)
- Keeps track of the repeating tone that is playing, automatically resumes after a momentary tone is done playing.
- Removes tones that aren't associated with actions (copter actions, user actions)
- Adds tones that are associated with actions (e.g. flight mode changes, autotune complete)

Todo:
- Needs a general "failsafe" tone.
- Needs a failsafe LAND warning tone
- Users want a "lost copter beacon"