Issue 2946: let Lyric_combine_music_iterator only listen to rhythmic events

Issue 2946: let Lyric_combine_music_iterator only listen to rhythmic events

Listening to articulations makes the rhythmic_music_iterator assume it
can just broadcast things like string numbers and right fingerings and
somebody will pick them up and deal with them which is, unfortunately,
not the case.

Listening only to rhythmic events should cure that problem.