* 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`
With this, add/remove buttons for displacement constraint will put the user in a
selection mode if pressed when nothing is selected.
Superclass `TaskFemConstraintOnBoundary` Allows code reuse for other
constraints. We do not need this in `TaskFemConstraints` because it is only used
in a subset of constraints, particularly that applies on only some of the
boundary entities.
TODO: Confirm Undo/Redo works appropriately.
TODO: Optimize `onSelectionChanged()`.
