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
The problem is caused by conflicting values of the anonymous enum of ViewProviderSubShapeBinder. The solution is to set a higher value than the highest value of EditMode
* add action to context-menu to remove an element
* use an exclusive button group to avoid to have several buttons pressed at the same time
* allow to completely clear the support
* when pressing OK/Cancel then operate on the document the shape binder is part of (which isn't necessarily the active document)
* remove obsolete parameter from ViewProviderShapeBinder::highlightReferences()
