gui: Add .zsav support in File|Open and File|Save As dialogs.

[pspp] / lib / gtk-contrib / README

This is not part of the GNU PSPP program, but is used with GNU PSPP.

This directory contains a version of the GtkXPaned widget.  It includes
minor modifications. GtkXPaned is licensed under the GNU Lesser
General Public License.  See COPYING.LESSER.