Implemented Session.after_redraw_message property.

NEW: set the property to display a message after the next redraw.

This allows for confirmation after control passes back up from delegate to
delegator.

Example:

    Abjad IDE - materials - test list (Bača) (EDIT)

       1: 17
       2: 18
       3: 19
       4: 20

          elements - add (add)
          elements - move (mv)
          elements - remove (rm)
          editing - done (done)

    Element]> done!

    Abjad IDE - materials - test list (Bača)

       1: __init__.py
       2: __metadata__.py
       3: definition.py
       4: versions/

          definition.py - autoedit (da)
          definition.py - check (dc)
          definition.py - open (do)
          definition.py - output (dp)

    Wrote list to /Users/trevorbaca/Documents/baca/score_manager_library/materials/test_list/definition.py.
    >