EventListener "priv" field

Dear Gdome2 users, Paolo in particular,

after some struggling with the Ocaml binding for Gdome2 we realized that
not only the "deprecated" field priv in an EventListener is necessary, but
also that it is not enough in general. There are cases in which the
pointer you store in "priv" is another reference counted object. Now,
whenever the EventListener is destroyed, you can't simply forget about
"priv", you also need to unref it.

One possible way to do this would be to allow not only storing the
"priv" field, but also a callback function that would be called just
before the EventListener is destroyed when its ref counter reaches 0.
DOM level 3 basically adds this functionality in a more general way, but
I believe it is not our intention to move to level 3 in the short term, so
we were wondering if the simple callback solution would meet your
consensus or not. We also understand that this would not be compliant to
the standard, but as an extension it doesn't affect anything else in the
API.

Concretely, the proposal is to have a separate constructor method for
EventListener that takes both the priv field and also the callback
function, which would make the API backward compatible.

In case nobody objects, I volunteer for doing the required modfications
(very few actually).

-- luca