articulate grace notes with time stealing

articulate grace notes with time stealing

This change makes \articulate handle grace notes itself, rendering them
to ordinary notes.  There are a couple of tweakable parameters controlling
the rendering.

This prevents \articulate causing the many "going back in MIDI time"
errors that it used to.  (Inserting a short rest after each note makes it
way too easy for following grace notes to need to steal more time from
the preceding rhythmic event than it has.)  In fact, when such errors
occur in the absence of \articulate, \articulate can now fix them.