validateConnection(cellViewS, magnetS, cellViewT, magnetT, end, linkView) - decide whether to allow or disallow a connection between the source view/magnet (cellViewS/magnetS) and target view/magnet (cellViewT/magnetT). end is either "source" or "target" and tells which end of the link is being dragged. This is useful for defining whether, for example, a link starting in a port POut of element A can lead to a port PIn of elmement B. By default, all linkings are allowed.