bugfixes, improvements

* document outline orientation change autoresize
* fixed pages being added twice
* thin border in the center of double pages
* double tap to zoom zooms out on second double tap
* some experiments on link detection on double pages, unfortunately not
successful
improvements

* single pages in double page modes now scaled and positioned correctly
* deactivated unavailable segments of toolbar switch in
thumbnail/outline view
* some preparations for restoring last selected mode (thumbnail or
outline)