workflows: correspond object state names with docs

* Fix reference to `completed_objects` in the documentation (was:
  `final_objects` in code).

* Expose all other states via the `BibWorkflowEngine` interface and
  expand documentation.
workflows: rename `ObjectVersion` FINAL to COMPLETED

* Rename `ObjectVersion` FINAL to COMPLETED for consistency with engine
  states and mid-level-API naming.

* Set `DeprecationWarning` on COMPLETED via new helper function.

* Introduce `staticproperty` and `classproperty` decorators.