Make "go to previous page" action restore original scroll position. The effect is that the clicked link ends up in the same position it was in before clicking, making it easy to pick up reading where one left off.
Closes #1458
This takes the approach of inserting a synthetic Link into the history list that points back to the first page visible in the viewport (based on view->start_page), guaranteeing that the document is in the exact same position on return as it was when the link is clicked.