Add gimbal to external control signal

Move PreflightCheckList to custom builds only. The model, which used to be implemented within BuiltinPreFlightCheckModel is now within the control itself. It is up to the implementer to define a proper model.

New qml base log replay status bar

Switch to using firmware manifest for ArduPilot flashing

Update ArduPilot version to latest releases only

Add support for geofence return point. Also fixed a bunch of bugs in geofence and rally point support.

Restore QGC to its normal UI layout (buttons on toolbar)

Offline Maps: Allow custom build settings overrides

Get rid of QGCView, QGCViewPanel and FactPanel. All messages and dialogs are now handled in the root window.

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