Change in visibility was causing combo box contents to reset. Visible
goes to false even when you move away from Plan view, not just when the
combo is unused.

- Incorrect signaling causing dirty but to be incorrect
- Remove all not working
- Min fence point count incorrect

This gets it above your head

Moving source files into Vehicles as it is a more coherent place.

Forcing QGC to use Mavlink V2 if vehicle supports it.

This makes it available on mobile builds

Added a second polygon. This polygon is updated from the first polygon at each mouse click and is what will ultimately be saved upon completion of drawing.