Let rhythmic-engraver make its articulation-or-event decision based on current listeners

Let rhythmic-engraver make its articulation-or-event decision based on current listeners

This removes the dependency of the rhythmic engraver on a static list
of unlistened event classes and thus is part of work on issue 2449.

As one effect, string numbers on isolated notes in Voice contexts are
now typeset by default since the string numbers have no listener in
Voice contexts even though they would have one in TabVoice.