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

Download Speed Tweaks (+2 squashed commits)
Squashed commits:
[ff0ef7e] Cleanup (remove OpenStreetMap)
[370fa1d] Done (+25 squashed commits)
Squashed commits:
[9f3368f] Need to check how many instances of MapEngine are active.
[d557eac] Download Optimization
[392f50a] Almost there.
Trying to optimize download.
[a49d1d2] Settings
[a94be97] Mostly there. Needs to finish options and map thumbnail.
[38d5a0b] Downloading tiles and deleting sets
[81101b9] More UI Work
[c597d4b] Downloading tiles
[a815e35] Get rid of OpenMaps
[7e177ea] More reorg
[fa6b671] Start handling create tile set
Fix signal order when creating fetch tile task
[2a31f4d] Refactoring
[268b906] Renaming things
[947d66e] Fix resource load error.
[19e2de8] Adding MapBox
[c73e627] Preparing download
[87bbf22] UI Tweaks
[3c32a86] A lot of UI code done.
[ece8ce2] Starting to deal with tiles
[7f387bc] Save tile set
[f66f343] Adding more code
[4de3418] Working
[f0cc25d] Done for the night.
[2d1d86e] Added SQL database for holding out own tile cache.
[d405a87] Convert to camel case as this has been driving me nuts.

Also added Center to Mission support