LP#1086458: add class to manage event listeners

LP#1086458: add class to manage event listeners

EventListenerList allows one to maintain a list
of event listeners, then remove them all when it's
time to clean up a window.

Usage is:

var list = new EventListenerList();
// attach an event listener
list.add(node, 'command', function(ev) { alert('BOO!'); }, false);
...
// get rid of them
list.removeAll();

Based on an idea by Jason Etheridge.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>