Optimizations of the Thumbnail list.

We now only create thumbnail widgets for thumbnails that are really visible in the viewport. That means we need to create only a very small number of widgets, and not one widget per page as before.

This gives a huge performance boost for all operations, expecially loading and closing of documents, and rotating the view, since the creation and especially deletion of QWidgets is quite slow.

The performance gain is only noticable for very large documents with 1000 or more pages.

There are still a few little regressions, which I will fix later.