Changes from MusiXTeX 120 to 121:

  + adjusted the offsets and/or vertical positions for some tremolos

  + moved \lastbarpos\lin@pos after calls to \Writ@meters 
    to correct \centerbar positioning

  + copied the definition of \Hpause from musixlit.tex

  + introduced \Changeclefs, \cHpause and \centerHpause

  + replaced \smalltype\it by \txtfont in octave clef commands 

  + moved definitions of \txtfont into \comput@fonts

  + added support for piano-pedal bracket notation
