FlightGear - Flight Simulator FGAddon

(merged from the 3.6 release branch)
* allow-toggle-cockpit := true (toggle with 'c' as usual)
* autopilot: in altitude hold mode, use indicated altitude, not
/position/altitude-ft.
* cockpit: in the navigation display, change the heading bug knob to use a knob
animation instead of two pick animations and some Nasal code.
* add a rating.