New navigator

New mission handling application "navigator".
 - Navigation state machine moved from "commander" to "navigator" app, it's responsive for publishing vehicle_control_mode and mission_item_triplet topics used by position controllers.
 - RTL mode implemented: climb, go to home position horizontally, fast descend, slow land
 - Mode switching by mavlink commands or RC (overrides commands if corresponding switches mapped)
 - Force vertical takeoff for multirotors, even if first waypoint is not TAKEOFF
 - Suport for landing/takeoff in the middle of mission (but not for FW, I think :) )

TODO:
 - RC failsafe
 - RTL: wait before landing (?)

WIP, not for merging yet, but for discussion.