The future patch will introduce Part::getTopoShape() to construct a
compound shape from a group. It will rely on the children visibility to
determine whether to include the child shape or not. This patch adds
children visibility tracking capability to group, and makes sure that
the group object will be marked for recomputation in case of any change
in group member, and their visibility status.
* Remove Prop_Output from 'Group' property.
* Added hidden property _GroupTouched to help propagate children change.
* Track children visibility change using signal
* GeoFeatureGroupExtension uses new PropertyLinkBase interface for
scope checking.
FreeCADs property system utilises some pointer math to calculate the offset between
property and base class. Due to virtual inheritance of th ePropertyContainer the memory
layout has been changed to rather random, which has lead to crashes dependend on the
order of object initialisation.
The solution is to not make PropertyContaner virtual but a class below, Base::Persitance.
Then the memory layout is random for Persistance, but it is perfectly aligned for the
base class chains from PropertyContainer onwards as well as from Extension onwards.
Hence the proeprty system was changed to take the offset always from those two.
