Add ability to print in number-up mode for file backend and lpr backend

GtkPrintOperation is now able to render multiple pages per sheet by its
own. The most important changes are in these functions:
  * increment_page_sequence
  * prepare_data
  * common_render_page
  * print_pages_idle
Patch also changes set of choices for 2 pages per sheet mode when
landscape orientation is used to "Top to bottom" and "Bottom to top".

gtk/gtkprintjob.c

@@ -205,6 +205,8 @@ gtk_print_job_init (GtkPrintJob *job)
   job->scale = 1.0;
   job->page_set = GTK_PAGE_SET_ALL;
   job->rotate_to_orientation = FALSE;
+  job->number_up = 1;
+  job->number_up_layout = GTK_NUMBER_UP_LAYOUT_LEFT_TO_RIGHT_TOP_TO_BOTTOM;
 }