Better handle the wait for opening windows

[whimboo]

Currently we have code like this:

       return self.wait.until(
            expected.new_browser_window_is_opened(
                self.selenium, handles_before),

Problem is that each consumer of the expected method would have to retrieve the handles before.

I propose that we get rid of this expected method, and replace it with whether a context manager, or having a callback. In both situations the wait will happen totally transparent to the consumer.

[b4handjr]

I personally like the idea of a context manager. I think that would help when we add different window types.