Split mark events into two classes
One class represents a true rehearsal mark and the other represents the (ab)use of a RehearsalMark grob to position arbitrary markup.
For true rehearsal marks, limit the 'label property to an index. Set 'text directly for ad-hoc marks.
Mark_engraver still listens for a common mark event, but enhancements are planned.
This is a more conservative version of the patch in Issue #5301 (closed).