不论鼠标指针穿过被选元素或其子元素，都会触发 mouseover 事件。
只有在鼠标指针穿过被选元素时，才会触发 mouseenter 事件
wrap, mouseover
count:
1
2
3
wrap, mouseenter
count:
1
2
3
wrap, mouseout
count:
1
2
3
wrap, mouseleave
count:
1
2
3
wrap, emulate mouseenter,用mouseover模拟实现mouseenter
count:
1
2
3
wrap, emulate mouseenter,用mouseout模拟实现mouseleave
count:
1
2
3