Event listener to extract (some) music events. (issue4373046)

[percival . music . ca]

Reviewers: ,

Message:
First version of Vivi event listener.  This is my first attempt at
non-trivial scheme work; please be gentle.  I fully expect to take half
a dozen revisions before it's ok to push.


Description:
Event listener to extract (some) music events.

This may be useful on its own, or as a basis for other people to
create/modify their own event listeners.

I considered a few different names; I'm not sold on
event-listener.ly.  Other contenders were staff-events.ly,
vivi-events.ly, and vivi-notes.ly.  In the end, I thought that it
would be better to avoid a Vivi-specific name.  Even though this
file is closely tied to Vivi, I don't think it's inappropriate to
include it -- we include the Festival song stuff, after all.

Please review this at http://codereview.appspot.com/4373046/

Affected files:
  A input/regression/event-listener-output.ly
  A ly/event-listener.ly