libview: Allow printing to all formats supported by backend

Set output format of EvPrintOperationExport according to the format
requested by user or by capabilities of used backend if there
is only 1 choice.

https://bugzilla.gnome.org/show_bug.cgi?id=724044
libview: Show correct warning when printing

Show that requested format is not supported by printer
if it is not supported.

https://bugzilla.gnome.org/show_bug.cgi?id=724044