Sometimes, the page author would like to cancel a client side event listener.
currently, there is no way to do this - you would have to instead code the
listening function to stub out if some value were set, instead, or overwrite the
function with a noop. That's awkward.
Instead, it would be desired that the listener be removed from the list of
listeners that the JSF client library keeps.
One way to do this would be to have the addListener function return a token, and
have a separate, cancel(token) function which would remove the listener
represented by that token.