Currently blendfile.py does not know how to handle embeded structs - this is solvable for later, but at least
do not raise exception about that when using values()/items() helpers of BlendFileBlock
(for now, return Ellipsis in those cases).
Also, added some user_data member to BlendFileBlock and DNAStruct classes, handy when you need to store
some own data for each of those, and want to avoid having your own list or dict for that.
