UI: Tree should keep scroll-state when ajax-call is made

Example: First the tree is expanded untill a scrollbar appears (Afbeelding 2/ Afbeelding 3). Then, we scroll down (Afbeelding 4/ Afbeelding 5). When you click on a node that is only visible after scrolling, the ajax-update will redraw the tree and the node you clicked on is not visible because the scrollbar is in it's initial state again (Afbeelding 6).