Add support for multi-note tremolo

Add support for multi-note tremolo

-) Remove all checks for exactly one or two notes as arguments to a
   tremolo repeat
-) Create proper beaming for multi-note tremolo (so far, the
   chord-tremolo-engraver always assumed exactly two beamed notes)
-) Extend the make-repeat function to scale the notes properly also
   in the case of multi-note tremolos

patch from issue 1786041 on codereview.appspot.com