Add ability to cancel a client side listener

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.
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.0]

Start by locating the JSF client library's addListener entry point and the list of listeners it maintains. Review how listeners are registered and determine how a returned token and cancel operation would identify and remove one. Done means a client-side listener can be registered, canceled, and no longer receive events.

Written by the indexing model from the issue text.