Use the previously added superclass `TaskConstraintOnBoundary` for more
constraints where it's usable.
Currently done:
TaskFemConstraintDisplacement
TaskFemConstraintForce
TaskFemConstraintSpring
TaskFemConstraintFluidBoundary
TaskFemConstraintHeatflux
TaskFemConstraintFixed
TaskFemConstraintTemperature
TaskFemConstraintPressure
Too different:
TaskFemConstraintTransform (single selection)
TaskFemConstraintContact (single selection, two fields)
TaskFemConstraintPlaneRotation (single selection)
(Other constraint tasks, which do not use `addToSelection`)
