Improved selector menuing.

Example:

    Abjad IDE - stylesheets

       1: clean-letter-14.ily (Abjad)
       2: clean-letter-16.ily (Abjad)
       3: rhythm-letter-16.ily (Abjad)
       4: time-signature-context.ily (Abjad)
       5: fancy-aracilik-style.ily (Aracılık)
       6: baca-letter-14.ily (Bača)
       7: gebiete-miscellaneous.ily (Gebiete)
       8: manos-header.ily (Las manos mágicas)
       9: manos-layout.ily (Las manos mágicas)
      10: manos-misc.ily (Las manos mágicas)
      11: manos-music.ily (Las manos mágicas)
      12: manos-paper.ily (Las manos mágicas)
      13: meters.ily (Manifolds)
      14: poeme-parts-layout.ily (Poème récursif)
      15: poeme-parts-line-breaking.ily (Poème récursif)
      16: stylesheet-addendum.ily (Red Example Score)
      17: stylesheet.ily (Red Example Score)
      18: traiettorie-header.ily (Traiettorie inargentate)
      19: traiettorie-layout.ily (Traiettorie inargentate)
      20: traiettorie-miscellaneous.ily (Traiettorie inargentate)
      21: traiettorie-paper.ily (Traiettorie inargentate)

          files - copy (cp)
          files - new (new)
          files - remove (rm)
          files - rename (ren)

    > new

    Abjad IDE - stylesheets

      Select storehouse:

       1: My stylesheets
       2: Aracılık
       3: L'archipel du corps
       4: Betörung
       5: Čáry
       6: Chrysanthemums
       7: Mon seul désir
       8: Gebiete
       9: L'imaginare
      10: Lidércfény
      11: Manifolds
      12: Las manos mágicas
      13: Poème récursif
      14: Red Shift Hijinks
      15: Sekka
      16: Tack
      17: Territoires
      18: Traiettorie inargentate
      19: Zeit

    > q

The 'Select storehouse:' subtitle is new in this commit.

This keeps the menu title 'Abjad IDE - stylesheets' the same through the
make-new use case.