util-app, flatpak: Use new GNOME Settings app-id
Flatpaked Boxes allows for calling GNOME Control Center when the user doesn't grant permissions for the Boxes to run in the background (after closing the window) when a VM is marked to "Run in background". This is useful because the Applications panel in gnome-control-center allows for reseting the flatpak app permissions.
Ever since commit gnome-control-center@acd59aec, the gnome-control-center app id got renamed to org.gnome.Settings.
With this changes, Boxes will attempt to activate org.gnome.Settings over DBus and fallback to the old org.gnome.ControlCenter when the first fails.
Fixes #787 (closed)