Improved library breadcrumb indicators.

OLD:

    Abjad IDE - materials - talea rhythm maker (Bača)

       1: __init__.py
       2: __metadata__.py
       3: definition.py
       4: illustration.ly
       5: illustration.pdf
       6: output.py
       7: versions/

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

          illustration.ly - interpret (ili)
          illustration.ly - open (ilo)
          illustration.pdf - open (ipo)

          output.py - check (oc)
          output.py - illustrate (oi)
          output.py - open (oo)

    >

NEW:

    Abjad IDE - materials - library - talea rhythm maker

       1: __init__.py
       2: __metadata__.py
       3: definition.py
       4: illustration.ly
       5: illustration.pdf
       6: output.py
       7: versions/

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

          illustration.ly - interpret (ili)
          illustration.ly - open (ilo)
          illustration.pdf - open (ipo)

          output.py - check (oc)
          output.py - illustrate (oi)
          output.py - open (oo)

    >