point-and-click in SVG

[Graham Percival]

The archives show a few half-hearted attempts to get this working,
but no actual patches other than Mike's "lily dances" patch from
Dec 2010 which wasn't merged.

I've got a patch which adds point-and-click,
https://codereview.appspot.com/8273045
but it's sub-optimal at the moment because you need to click
slightly away from the note -- a directly click on the notehead
does nothing.  This is because output-svg.scm's
  (grob-cause ...)
is called *before* drawing the path of the notehead, and SVG has
no z-index.

The question is this: what calls grob-cause?  Ideally I'd like to
have an SVG-specific function which formats a note with
point-and-click info as:
  <a xlink="file.ly:2:3:4" />
    <path normal lilypond svg notehead />
  </a>
but failing that I'd be content to shove the <a...> below the
default notehead.  Does this require mucking about in C++?  And if
so, can this be done comfortably without having a ton of
backend-specific information there?  It would feel a lot cleaner
if I could change the formatting of an SVG note within
scm/framework-svg.scm and scm/output-svg.scm only.

- Graham