This reverts commit b0452667c72f330323f6b3366f45bf039d05feef, reversing
changes made to b2d99f3fa7b4c7c305aed4a8d975a8624a6d590f.
See forums discussion: https://forum.freecadweb.org/viewtopic.php?p=549120
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.
