- add fields to specify a formula for the displacement, this is necessary to make transient deformation analyses with Elmer
- add option to set displacement from result of flow equation
- this way also a major overhaul -> simpler dialog logic etc.
Minor UI improvement. The behavior had changed when the buttons were changed
from `QPushButton` to `QToolButton` in 64a95596459f8db6b5fce16a354913ac1d669344.
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()`.
