Add "Page.WaitStable" helper

[ysmood]

How it works is simple, polling the changes of the DOM tree, wait until the most of DOM tree doesn't change for some time, such as 95% and 300ms.

A similar function is Element.WaitStable.

[ysmood]

@shuiRong What do you think about this? I feel I will be more useful than Page.WaitElementsMoreThan.

[ysmood]

But it has a major issue. If a page has constant animation like a clock, this function will be useless. So we may have to add an option to use CSS selectors to ignore some areas.

Also, a page can use js to append elements after 10 seconds, which is impossible to detect automatically.

[ysmood]

For #556 we can use opencv to diff the image, such as when 95% is the same then we think the page is stable:

https://github.com/hybridgroup/gocv

https://stackoverflow.com/questions/27035672/cv-extract-differences-between-two-images

[moredure]

Would be cool.