Further GPS and route manager behavioural fixes

Further GPS and route manager behavioural fixes

* When the nav-radio is slaved, calculated radial/target-hdg-deg
 (needed by some autopilot logic)
* Handle editing (including deletion) of route waypoints correctly,
 including deleting the active waypoint
* Add a signal to the route manager when the last wpt is reached, and
 use it in the GPS to revert to OBS mode.
* Change the altitude handling to use the specified cruise altitude
* Fix a bug where autopilot/locks/altitude was treated as a boolean