add swipe helper so that you can dismiss the undo bar by swipe guesture

[jtbuaa]

if you do not wan't to wait 5 seconds for the undo bar dismiss itself, you can dismiss by swipe.
this swipehelper is from Android4.4.

[soarcn]

Hi @jtbuaa

Thanks for the PR, I just notice that google also introduce swipe to dismiss function to official snackbar. so I'd love to support this.

But your implementation introduce too many classes, which is not necessary from my view. I'd prefer keep it as simple as possible. I would suggest that using google snackbar's implementation for this. So I would close this PR now, and you are welcome to pull new request for this. Thanks

[jtbuaa]

that's ok. hope you support swipe to dismiss function soon.