Now that the org.gnome.Shell.Extensions interface exposes the
disable-user-extensions setting on D-Bus, we can use that instead
of the shell's GSettings.
In a future where we distribute the app separately as flatpak, this
will require one less hole in the sandbox.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1081
Using the "regular" loadInterfaceXML() helper means less code duplication,
but it also ties us to the resource used by gnome-shell.
In order to untangle the extension app from core gnome-shell, change that
to load the interface from the existing data resource instead. While that
does involve reimplementing loadInterfaceXML(), it's not too bad actually
with the resource-loading code stripped (as the data resource is already
loaded by the package module).
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1081
