NOTES

SOME RANDOM COMMENTS:
=====================

 * We assume that all documents can be broken down into a linear
   collection of pages.

 * If a document type doesn't break down in such a way (like web pages)
   then it's probably not a good fit for this application.

 * Each page has a natural page size in pixels.  This is generally
   ignored in favor of a scale-to-fit mode, but is occasionally
   important for backends like the image backend.

 * Each page is not necessarily the same size.

 * We refer to pages by page number.  This number ranges from 1 to
   document->n_pages.  A page index of -1 means the current set page,
   and a page index of 0 is not used.


THOUGHTS ON THREADING:
=====================

 * The primary thing we are trying to do is minimize switching pages, as
   doing so is slow for backends.  Additionally, some operations on the
   backend are slow, leaving poor interactivity.

THOUGHTS ON SELECTION:
=====================

 * On button_press, we record selection_start, and set in_selection.

 * selection_list must be ordered!!!

 * On motion_notify, we update selection_end and call compute_selection,
   that keeps an ordered list of the selections.  

 * If any of the selection changes, we notify the pixbuf_cache right
   away.

 * On button_release, we unset in_selection, and trim all the current
   pixbufs.

 * If a resize (somehow) occurs, during a selection, we clear
   in_selection and abort.

 * I'd like to support shift-click to handle extending the selection,
   but for that to survive resizing, I might need to store the points as
   doubles, etc.  It should be possible to reconstruct it from the
   existing EvViewSelection structs, so maybe I don't need it.

 * click-drag, dbl click, triple click, shift-click, search, shift-cursor move


THOUGHTS ON HISTORY:
====================

 * We need single history button, back/forward will complicate things.
 
 * When we jump on dest link we should add current page and link to the history.
 
 * When we jump on external link we should just add external link.
 
 * It's enough to have 7 entries in history, too many entries complicates things.
 
 * We should avoid duplicate entries in history. If we'll activate entry
   that already exists (have the same title) we can just move it to the top.