printing: Return correct width and height of page setup

Use correct margins when computing width and height returned by
gtk_page_setup_get_page_width() and
gtk_page_setup_get_page_height().

https://bugzilla.gnome.org/show_bug.cgi?id=671895
printing: Translate context to correct position

Use correct margins of given page setup when translating context
to imageable area.

https://bugzilla.gnome.org/show_bug.cgi?id=671895
Minor documentation fix