Flatpak integration

Merged Georges Basile Stavracas Neto requested to merge gbsneto/build-system-improvements into master

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:

new headerbar

[2] Flatpak warning:

flatpak warning

CC @hadess @bberg @gicmo

Edited by Georges Basile Stavracas Neto

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