Package NVDA for the Windows Store (#7851)
* Allow Building an appx package by running scons appx
* appveyor should also build the appx package
* Disable features that are incompatible with Windows Store policy
* Restart on Windows store updates
* Change publisher to match Windows store, and set appx version so minor is build and revision is 0
* Hardcode certain Windows Store publisher details, and don't sign the appx anymore (Windows Store does that).
* Change back the publisher to NV Access in appveyor.yml. It is now hardcoded separately in the appx manifest for windows store.
* scons appx now produces two appx files:
* storeSubmition: not signed, and a publisher ID matching NV Access Limited's Store publisher ID. Suitable for submitting to the Windows Store via NV Access Limited's developer account
* sideLoadable: signed by NV Access Limited, suitable for installing on a Windows 10 system manually as a side-loaded app for testing.
