_project.py: Adhere to policy on private symbols

And adjust all surrounding sources for changed symbols.

Additional details:

  o Added Project.get_shell_config() to report the shell configuration,
    instead of making those members all public

  o Moved assertions about project specified format versions required
    of plugins out of Plugin.__init__, and into Project.create_element()
    and Project.create_source(), so we can keep more things private

This is a part of issue #285