The number of pages that will be printed.
Note that this value is set during print preparation phase (gtk_print_status_preparing), so this value
should never be get before the data generation phase (gtk_print_status_generating_data). You can connect
to the [[email protected]:GtkPrintOperation:status-changed] signal and call [
[email protected]_n_pages_to_print] when print status is gtk_print_status_generating_data.
This is typically used to track the progress of print operation.