Copperbox revision 3210.
I've added Timeline into the Context which is threaded through all the monads (Advance, EventList, Score..).
Because the global timeline allows tempo and time signature changes, Context no longer supports a simple API to get quarter_note_length (quarter_note_length is now dependent on position in the score). Thus, the note lists used by Demo02 and Demo03 are broken again, pending a rethink.