* lily/include/music.hh (class Music): unvirtualize transpose().

* lily/include/music.hh (class Music): unvirtualize transpose().

* lily/sequential-music.cc: remove file.

* lily/include/music.hh (class Music): replace Music::start_mom()
by start-callback property

* lily/time-scaled-music.cc: remove file.

* lily/include/music.hh (class Music): include SCM init argument.
(class Music): replace Music::get_length() virtual by
length-callback property everywhere.