"Professional CMake" book suggest the following:
"Targets should build successfully with or without compiler support for precompiled headers. It
should be considered an optimization, not a requirement. In particular, do not explicitly include a
precompile header (e.g. stdafx.h) in the source code, let CMake force-include an automatically
generated precompile header on the compiler command line instead. This is more portable across
the major compilers and is likely to be easier to maintain. It will also avoid warnings being
generated from certain code checking tools like iwyu (include what you use)."
Therefore, removed the "#include <PreCompiled.h>" from sources, also
there is no need for the "#ifdef _PreComp_" anymore
* 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()`.
