Benoît Knecht authored Mar 23, 2013 and Sebastian Ramacher committed Mar 23, 2013

Instead of guesstimating the values of the scrollbars adjustments after
a change in zoom level, connect callbacks to the "changed" GtkAdjustment
event (which is emitted when the bounds or page_size of the adjustment
change, e.g. when the zoom level changes), and compute the new values
from there.

The previous adjustment values are tracked in zathura->ui.hadjustment
and zathura->ui.vadjustment (and updated by signal handlers as well), so
that the view's position can be maintained while zooming.

cb_view_hadjustment_changed() centers the page horizontally if a
"best-fit" or "width" zoom is being performed, or if "zoom-center" is
true; otherwise, it keeps the view horizontally centered around the same
area of the page.

cb_view_vadjustment_changed() always keeps the view vertically centered
around the same area of the page.

Many thanks to Marwan Tanager for thoroughly reviewing the various
stages of this patch, and actually coming up with a working solution.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>