use PointerEvents to prevent

1. add beforeClick event to flatButton
2. add setPointerEvents function
3. set the pointerEvents properties of cancelButton and confirmButton to 'auto' before page enter
4. set to 'none' when another button is clicked
`setPointerEvents` 这个函数的逻辑可以放到 FlatButton 里面，这样调用时的的接口就可以是：

```coffeescript
confirmButton.setPointerEvents 'auto'
cancelButton.setPointerEvents 'none'

```