Tag model entities with their "object_type"
this aims at preventing constant usage of isinstance() based dispatch
code when writing generic code handling model entities.
For example, the "object_type" argument of JournalWriter.write_addition() has
become superflous now we only pass model entities, etc.
This idea comes olasd's reading of mypy doc:
This comes with a refactoring of from_dict.DiskBackedContent to make
it *not* inherit from model.Content: object_type being Final, it cannot