issue #2087: Modify singleton implement for EventDispatcher,  adding EventDispatcher::destroyInstance().
issue #2087: Invokes EventDispatcher::destroyInstance() when purging Director.
issue #2087: Small improvement in Node::removeAllEventListeners. Adding Node::_oldEventPriority property.
Merge pull request #3681 from dumganhar/iss2087-new-event-dispatcher

issue #2087: Small fix for EventDispatcher. Adding EventDispatcher::destroyInstance method.[ci skip]