Re: Implement define-event-function (issue 5083045)

[dak]

If this can be verified as working, it might go on the patch countdown,
barring comments that require addressing.

Note that ly:event? is not part of the public API of
define-event-function: if ly:event?'s function name changes at some
later point of time, uses of define-event-function will not need to
adapt.

http://codereview.appspot.com/5083045/