add functions to compute page positions
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.