Changes from MusiXTeX 123 to 124:

  + preserve changes to \transpose in an hboxed tremolo command
  + test that musixtex.tex has already been loaded in some extensions: 
  +   musixps, musixec, musixhv, musixplt, musixtmr, musixppff 
  + changed from \it to \ppff in several places
  + support new command \setinterstaff{n}{m}
  + define \largenotesize, \Largenotesize
  + corrected typos \twtybf[g] \svtbf[g]