gnome-session moved away from using properties over DBus in 2008, which
means that the code in GNOME 3.0 never should have worked -- but it did,
which makes me suspect that it was a quirk of the GJS DBus implementation.
Switch over to the proper inhibitor API, which is based on methods. If
gnome-session eventually gets ported to GDBus, then we can switch back
to properties.
https://bugzilla.gnome.org/show_bug.cgi?id=672270
09607787cc