Create new, specific button for clearing mission from vehicle
Adjust sizing of DropPanel

* Added support for sat count to Sensors check control

Automated preflight checklist: Initial commit. Note that this requires an autopilot firmware that writes the respective health flags in the SYS_STATUS mavlink message

Adding missing qgcView to QGCMapPolygonVisuals instance in SurveyMapVisual.qml

Previous mechanism was exposing possible bug in Qt which would prevent
clicks from work on ToolStrip buttons after cancel MouseArea was hidden.