GTK PrintToFile settings

GTK PrintToFile settings

To make setting output directory and filename simpler in the PrintToFile
dialog two gtkprintsettings have been added GTK_PRINT_SETTINGS_OUTPUT_DIR
 and GTK_PRINT_SETTINGS_OUTPUT_BASENAME.

This will reduce the code needed to implement a better name than "output.pdf"
and actually makes more sense than the existing setting
GTK_PRINT_SETTINGS_OUTPUT_URI which doesn't work seamlessly with
GTK_PRINT_SETTINGS_OUTPUT_FILE_FORMAT like the new settings do.

https://bugzilla.gnome.org/show_bug.cgi?id=657322