The plan is to put in adjustment.c every piece of code that has to do
with document positioning, either computing it from data on the document
side, or talking to GTK.

We want to have at our disposal functions to compute sizes and positions
without having to ask for it to a GTK widget. The new functions are:

 - move page_calc_height_width to adjustment.c

 - add page_calc_position that rotates a position relative to a page
   according to the rotation settings.

 - add position_to_page_number that computes the number of a page
   sitting at a given position (given in document-relative coordinates)

 - add page_number_to_position that computes the position (in document
   relative coordinates) that will be placed at the center of the viewport.

 - add page_is_visible that checks whether the given page intersects the
   viewport.