Remove performance overhead of taking screenshots

[Galad]

Currently, taking a screenshot saves it directly to a file.
This cost a lot of time, and it is not often necessary to save them to a file when the test pass.

Selenium screenshots can remain in memory until it is decided by the user that it needs them.

In that effect, we would have an additional overload of TakeScreenshot:

actor.TakeScreenshot(@"C:\Screenshots", "Test1", out var saveToFileObserver);
actor.When(Click.On(button));
actor.When(Click.On(otherButton));
// screenshots are not saved on the disk
saveToFileObserver.OnNext(Unit.Default);
// screenshots are saved

[Galad]

The final version takes an observer instead of returning one:

actor.TakeScreenshot("Test1", new SaveScreenshotsToFileOnComplete(@"C:\Screenshots"));
actor.When(Click.On(button));
actor.When(Click.On(otherButton));
// screenshots are not saved on the disk
saveToFileObserver.OnComplete();
// screenshots are saved