Support articulations and breaths in MIDI. (issue 3664)

Support articulations and breaths in MIDI. (issue 3664)

Articulations and breaths can alter the length and volume of the note.
(Breaths affect the previous chord.) This is controlled by their
midi-length and midi-extra-velocity properties. The standard articulations
now have these properties where appropriate.

(seconds->moment s context) is a convenience function for making
absolute durations.