During undo/redo the order in which objects are removed from a group and added to annother is arbitrary, hence it cannot comply to the single Group rule. Not echking for it in this case is unproblematic, as undo/redo only recrete former states, and those states have been checked in normal operation.
This is the first feature that used GeoFeatureGroupExtension and required links to the groups inside as well as to things on the same level. Hence a few modifications to link scopes have been nesseccary.
Due to problems onthe windows platform the virtual inheritance approach must be dropped. NExt to the already reimplemented proeprty interface the Type interface is reimplemented too. This change allows to revert some earlier changes.
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.
