VtolPathFollower: makes RTH altitude and RTH velocity adjustable in settings
VtolPathFollower: emergency land when gps data unuseable for navigation

Gives some predictable behavior to VTOL path follower in case of
GPS signal lost (or unusable quality). When GPS lost, it will wait
and continue operations for 2 seconds to ignore short-term GPS glitches
and then emergency landing is enabled. In emergency landing VTOL attitude
is stabilized to be level and throttle is controlled to achieve
configured landing  vertical speed.
failsafe: make failsafe behavior configurable
failsafe: failsafe_control can now switch to RTH flight mode

If configured, Return To Home flight mode is selected when RC transmitter
signal is lost. There is protection against accidental RTH activation
when last known throttle input was 0. It also do some configuration
and alarms sanity checks before activation and when it failed, old-style
"throttle cut" failsafe kicks in.

GPS signal quality is evaluated by VtolPathFollower itself and in case
of low-quality or no signal, it renounce horizontal control, set level attitude
and start descent immediately at configured landingRate velocity.
failsafe: fixed uninitialized variable
failsafe: updated warning message
Merge branch 'failsafe' into draco_experimental

Conflicts:
	ground/gcs/src/plugins/config/configinputwidget.cpp