node常用属性:
1._eventDispatcher:事件派发器，使用它来管理当前节点（Scene, Layer和Sprite）不同事件的分配。 _eventDispatcher有两个添加监听器的方法:
// Adds event listener
/** Adds a event listener for a specified event with the priority of scene graph.
* @param listener The listener of a specified event.
* @param node The priority of the listener is based on the draw order of this node.
* @note The priority of scene graph will be fixed value 0. So the order of listener item
* in the vector will be ‘ <0, scene graph (0 priority), >0‘.
*/
void addEventListenerWithSceneGraphPriority(EventListener* listener, Node* node);
/** Adds a event listener for a specified event with the fixed priority.
* @param listener The listener of a specified event.
* @param fixedPriority The fixed priority of the listener.
* @note A lower priority will be called before the ones that have a higher value.
* 0 priority is forbidden for fixed priority since it‘s used for scene graph based priority.
*/
void addEventListenerWithFixedPriority(EventListener* listener, int fixedPriority);
事件派发器由事件的优先权来派发事件。 方法一：由node的_localZOrder值的高低派发，值高优先，当这个node被移除时，该监听器也会被移除。 方法二：由整型值的高低派发， 当这个node被移除时，该监听器需要自己手动移除
