CCEventDispatcher crash fix
Fix possible crashes which could be caused by the EventDispatcher having listeners associated with nodes are destroyed. Catch the case where a node registers a listener while we are dispatching events and gets destroyed while that event is still being dispatched. Check the list of nodes to be added into the event dispatcher fully when we are cleaning listeners for a target node.

This issue was found using the extra debug verification in this pull request:
https://github.com/cocos2d/cocos2d-x/pull/6011
Add an event dispatcher test
Add an event dispatcher test to help reproduce crashes fixed by the previous commit and to verify that the fix still works.
Unregistered listener when it's removed from _toAddedListeners.
Merge pull request #6162 from dumganhar/pull/6160

Fixed a potential crash in EventDispatcher, based on PR #6160
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