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

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".