Copter: increase GPS_HDOP_GOOD default to 2.3
AP_Notify: Rework ToneAlarm_PX4 to play tones from local array
Copter: play various tones
Notify: add comments re events
ToneAlarm_PX4: add comments
Notify: rename notify_types to notify_flags_type
Copter: minor format change
Notify: rename failsafe_action event
Copter: set notify event when failsafe changes flight mode
ToneAlarm_PX4: remove user_mode_init flag

Replaced by an initialised check in the main ArduCopter flight code
Copter: do not send mode_change event before init complete

This avoids a beep as the flight mode is set during start-up
Notify: minor event reordering

No functional change
Copter: allow radio failsafe while disarmed so that user can be notified
AP_SpdHgtControl: added get_land_sinkrate()
AP_TECS: added get_land_sinkrate()
Plane: improve landing aim point calculation

try to account for wind and current ground speed to dynamically update
glide slope in landing to flare at the right point