Added 'wait_for_page' to WebAppUI

I tried having one page wait for another, but ran into a problem.  When I do this:

    EmptyPromise(self.ui['page_name'].is_browser_on_page, "got to next page")

The `self.ui['page_name']` throws a `WrongPageError` because it's not on the right page :)

I've added a `wait_for_page` method to `WebAppUI` to work around this, since I think this will be a common pattern.

@jzoldak