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

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.