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

Also distinguish Emergency Stop better

The idea here is to prevent people from accidentally flashing the wrong firmware

* Added support for sat count to Sensors check control

* QGCCheckListItem -> PreFlightCheckButton
* CheckList -> PreFlightCheckList

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

For much better performance

This forces a commit on any uncommitted TextFields

Added (UI) support for vehicles exposing more than one camera.

Create a "Dead Mouse Area" control to eat mouse events underneath desired area. This is to prevent mouse events from leaking through controls underneath the area.
Replace original "NoMouseThroughRectangle" as this is more versatile.
Prevent mouse through events from leaking underneath the Fly View tool strip.