Around 2.91.90, gnome-session-save was renamed to gnome-session-quit.
This commit restores compatibility with the older gnome-session, for
those testing under GNOME 2.32 or below, by calling the DBus methods
directly.
https://bugzilla.gnome.org/show_bug.cgi?id=644591
The DBus JS binding will complain if the signature of a method
or of a signal is undefined, so we need to define it even if it
is an empty string, and we need to use the correct property name
for signals.
https://bugzilla.gnome.org/show_bug.cgi?id=643374
