* Now the behavior is consistent with behavior of PD fillets, for example.
See https://forum.freecadweb.org/viewtopic.php?f=18&t=67135#p580192. In the
future it may be possible to reuse some code from there.
* When the last selected item is removed we need to reset the existing
highlighting separately.
* Use `Gui::ButtonGroup` in `TaskFemConstraintOnBoundary`