ArduPilot for configurable mavlink stream rates

Support for new AltitudeFactTextField

Expose guided altitude settings. Rearrange Fly and Plan view settings.

Add new Help page under Settings

* QGCCheckListItem -> PreFlightCheckButton
* CheckList -> PreFlightCheckList

The LinkIndicator shows the connected links and lets the user select the priority link,

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