`blendfile.py`: add basic support for non-DNA blocks (allocated strings...).

Allocated strings or arrays of numbers have no DNA struct, they are
'raw' blocks of data in blendfiles.

Reading those was not supported so far, this commit adds basic support
for them, although user needs to know and manually call the code for the
relevant properties pointers.

Propper automated support could be added once T99875 is addressed.