updates: Automatically download updates rather than installing them

This is described in https://live.gnome.org/GnomeOS/Design/Whiteboards/SoftwareUpdates
with the iplementation described in http://freedesktop.org/wiki/Software/systemd/OSUpgrade

Note: This requires PackageKit 0.8.1 to work correctly.

plugins/updates/gsd-updates-manager.c

@@ -1346,6 +1346,9 @@ gsd_updates_manager_start (GsdUpdatesManager *manager,
        g_object_set (manager->priv->task,
                      "background", TRUE,
                      "interactive", FALSE,
#if PK_CHECK_VERSION(0,1,8)
                      "only-prepare", TRUE,
#endif
                      NULL);

        /* watch UDev for missing firmware */