Updating sizing

[Jonathan Blandford <jrb redhat com>]

I just committed a patch that turns "Fit width" and "Best fit" into
toggle buttons, as well as adds a small border to the document.  I think
it looks pretty good and it feels like the right behavior to me.  I am
a little concerned about the speed of the operation.  It doesn't resize
as smoothly as eog does right now, and feels really 'chunky' on complex
documents.  It could just be my computer (which is no speed deamon) and
the performance might be good enough as is, so I'd love other opinions.

If it isn't good enough, we're going to have to look into making evince
threaded.  I'm pretty convinced that we can't do continuous scrolling
without making it threaded anyway, so we'll have to go that route
eventually.  This is going to destabilize it initially, so we should
probably put it off until we can make a nice stable release, and make a
branch.

Thoughts?
-Jonathan