Flatpak integration
This MR introduces Flatpak support to GNOME Settings. The Flatpak builds are strictly meant for development purposes and greater integration with the extended GNOME stack (GNOME Builder, OSTree, etc).
As a small nicety over the Flatpak manifest, and to make sure people don't think this is supposed to work perfectly and is production ready, the main window now has a different style for development [1] builds and explicitly warns about running from Flatpak [2].
[1] New style:
[2] Flatpak warning:
