Make \balloonText into post-event, \balloonGrobText into proper event

Make \balloonText into post-event, \balloonGrobText into proper event

This is a stopgap measure for getting some consistency into music
function use.  The real fix would be to convert \ballonText and
\balloonGrobText to the same kind of interface that \footnote uses.
Indeed, it might be saner to retire the whole AnnotateOutputEvent
folderol and instead just use footnotes without footnote-text.