Support properties on articulations.

Support properties on articulations.

Shorthand articulations like -. can now be ArticulationEvents, not just
articulation names. Previously a shorthand created a new ArticulationEvent
with its name, so there was no way to give it properties. Now it can be (and
is, by default) an ArticulationEvent.

Note that redefining a shorthand to a string (e.g. dashDot = "staccatissimo")
still creates propertyless articulations, regardless of \staccatissimo.