Extended Editor.__init__().

You can now initialize (vanilla) Editor with an existing target.

Example:

    >>> session = scoremanager.core.Session(is_test=True)
    >>> clef = Clef('alto')
    >>> editor = scoremanager.editors.Editor(
        session=session,
        target=clef,
        )
    >>> editor._run()

    Clef

        1: name (nm): alto

        done (done)

    > nm

    Name> tenor

    Clef

        1: name (nm): tenor

        done (done)

    > done

    >>> editor.target
    Clef(name='tenor')

Removed Editor._edit() because this new extension provided to Editor.__init__()
makes Editor._edit() unnecessary.

MODEL: there's also an important change provided to
Editor._set_target_attribute() in this commit. The method now implements logic
to use new() to make changes to the editor target while editing. This is good
because the implementation of new() is powerful and because this brings us one
step closer to being able once again to edit composite objects like
rhythm-makers.