Refactor mark events ...
Split mark events into three classes:
* default rehearsal mark
* specific rehearsal mark
* ad-hoc mark
In anticipation of adding a fourth class with both a symbolic label
and text markup, stop using the 'label property to hold the markup of
ad-hoc marks. Use 'text instead.
Diff:
Passes make, make check and a full make doc.
I'll leave this on Review - there is a comment from David K on Rietvel.