Convert bool settings to SettingsFact

* Holds all the settings for the app
* Allows core plugin to override default values

Made QGCCorePlugin derive from QGCToolBox and expose everything through the same mechanism as everything else.

This makes it available on mobile builds

_finalize methods need information which is not available if all you
have is a mavlink_message_t

Move battery.percentRemainingAnnounce to Qml Global

The static Fact was causes problems with Qml

offline mission planning settings

more conversion stuff

some fixes and added area unit drop down

Conflicts:
	src/MissionEditor/MissionItemStatus.qml

removed comment

finish unit conversion

correcting typos

Note that a change in zoom signals a `zoomLevelChanged` event; this in turn triggers `FlightDisplayViewMap` which sets the property changed in this commit to
```
onZoomLevelChanged: QGroundControl.flightMapZoom = zoomLevel
```

This triggers the following message in Application Output: `onZoomLevelChanged: QGroundControl.flightMapZoom = zoomLevel`.

This was the case before and after this fix, I don't know if that's a real issue though

- Convert mission load/save to it
- Allow parameter load/save on mobile