No more user prompting

Convert bool settings to SettingsFact

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

* Fix minor spelling error

* Added support for the AUAV x2.1 Flight Controller
    also removed a duplicate entry for MindPX
    from src/comm/USBBoardInfo.json

Adds entries required to correctly identify, download and flash PX4
firmware to the AUAV X2.1 Flight Controller

* Update USBBoardInfo.json

Removed un-needed fallback entries. This has been tested!

And fix broken file path to save video on windows

- Shot Interval in Survey
- Much change to time/distance calc

Shows horizontal/vertical scroll indicators

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