bgo#683335 - Set the basename for print-to-file

Per https://live.gnome.org/GnomeGoals/PrintToFile

 , GTK+ 3.6 lets the application set a filename
for the print-to-file command so that GTK+ can present a better default than just 'output.pdf'.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>