Better structured top-level menu: examples, library, scores.

Example:

    Abjad IDE

        Example materials:                           16: baca-letter-14
           1: example articulation handler         Scores:
           2: example dynamic handler                17: aracilik
           3: example markup inventory               18: archipel
           4: example notes                          19: betoerung
           5: example numbers                        20: cary
           6: example pitch range inventory          21: chrysanthemums
           7: example sargasso measures              22: desir
        Example scores:                              23: gebiete
           8: blue example score                     24: imaginaire
           9: etude example score                    25: lidercfeny
          10: red example score                      26: mannigfaltigkeiten
        Example stylesheets:                         27: manos
          11: clean-letter-14                        28: recursif
          12: clean-letter-16                        29: red shift
          13: rhythm-letter-16                       30: sekka
          14: time-signature-context                 31: tack
        Library: materials:                          32: territoires
          15: baca numbers                           33: traiettorie
        Library: stylesheets:                        34: zeit

    >

Three main parts: example assets; (user) library; (user) scores.