added event `ClosingFinished` to `Flyout`

In different apps I create flyouts dynamically and add them to an `ObservableCollection<Flyout>`. In the main window I bind to that collection: `<Controls:FlyoutsControl ItemsSource="{Binding Flyouts}" />`. However, when closing a flyout, I have to create an asynchronous task waiting 500msec before removing the flyout from the collection. That's necessary because otherwise I wouldn't get the nice hiding animation. In this PR I've added an event to `Flyout` which is raised after the HideStoryboard completed. With this event I don't need to wait asynchronously but could delete the flyout when this event is raised.