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.
In the long time of parallel assembly development an own drag'n'drop interface has been added to master and assembly branch. After merge both have been available. This commit removes one and keeps the more extensive implementation
This one prevents thurther object structure damage if
DocumentObjectGroup::Group connains multiple links to the same feature.
Also minorr changes:
- Fix some spacing
- Ask the user if he wants to delete group content
