Much better error reporting for Mission transfer errors

When downloading mission items from the autopilot we need to retry more
agressively, so use a shorter timeout. If we don't do this, we might not
re-request a mission item in time and the autopilot has already given up
the overall mission transfer.

This fixes the case where a mission request from QGC to autopilot or a
mission item from autopilot to QGC gets lost on the link and needs to be
requested again.

By adding a member variable for the total mission item count, we can
actually correctly estimate the progress.

Also fixed bug with ArduPilot guided waypoint write which would trash
existing mission.

This time to handle correctly updating plan/fly view when plan changes

Works around ArduPilot strangeness with empty missions end

Fixed a pile of bugs as well

Retry in all the right/possible places. Don’t freak out about things
coming in unknown sequences.

Plus new unit tests

Also merged MissionController and MissionEditorController into single
MissionController implementation