* Documentation/user/refman.itely (More stanzas): document slur

* Documentation/user/refman.itely (More stanzas): document slur
deficiency.

* lily/melisma-performer.cc (try_music): new file. Handle
melismata to help lyrics.

* lily/beam-performer.cc (try_music): new file. Handle beam events
to help lyrics.

* lily/score-performer.cc (one_time_step): reinstate
process_music().

* lily/performer.cc (process_music): add to interface.

* lily/performer-group-performer.cc (process_music): new function.

* lily/dot-column.cc (do_shifts): do collision resolution before
dot positioning.