Adds the specified function to the list of functions to be called
when a key event occurs. The data element will be passed to the
AtkKeySnoopFunc (listener) as the func.data param, on notification.
[R object] a R object that points to a block of data that should be sent to the registered listeners,
along with the event notification, when it occurs.
Value
[numeric] added event listener id, or 0 on failure.