Navigator rewrite - DrTon

@LorenzMeier, I know that you don't like "big rewrites", but I want to try again :)

- Navigator API changed: now ready to move manual control from controllers to navigator, less code in navigator mode implementation classes
- More strict usage of MissionBlock and children - don't set position setpoint directly, use mission_item instead
- Takeoff on first waypoint implemented for multirotors
- Duplicated (x4 times) code removed in Mission class
- Fill "next" position setpoint in triplet
- Various fixes

Need more testing, especially on FW.