gtkapplication-dbus: use application id instead of prgname for SM

gtkapplication-dbus: use application id instead of prgname for SM

gnome-session takes an "app id" for client and inhibitor registration,
This app id is supposed to be a desktop file id (complete with the
.desktop extension), but gtk+ currently uses g_get_prgname ().

This commit changes gtkapplication to use the application id instead,
which is a much more natural fit.  gnome-session is going to be updated
to stop using the .desktop extension, too, so everything is consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=763106