Improves the use of the ShapeAppearance property for the Part workbench.
removes DiffuseColor property
adds Python compatibility using custom attributes
transitions DiffuseColor to ShapeAppearance on open
Improved UI elements for setting object appearance, and appearance per face
Lays the foundation for future texture support
Uses new material system for appearance
Each feature object now has a property called ShapeMaterial that
describes its physical properties. If it has a shape, it has a
material.
The ShapeColor attribute is replaced by a ShapeAppearance attribute.
This is a material list that describes all appearance properties, not
just diffuse color. As a list in can be used for all elements of a
shape, such as edges and faces.
A new widget is provided to allow the user to select materials in a
consistent fashion. It can also launch the material editor with its
more advanced capabilities.
* 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`
