当用户长按地图时回调此方法，如果长按时触发某一个marker对象的拖动事件，则不会回调此方法。
This method is called back when the user long-presses the map. If the long-press triggers a drag event of a marker object, this method will not be called back.
参数:
point - 用户长按的地理坐标。
The geographic coordinates of the user's long press.