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:
