Work around android Qt bug where wrong virtual keyboard is displayed

Add flickable

Add file extension if needed

Duplication cleanup

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