`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.