[FR] Add "CustomBalloonClosed" event

[DominikPalo]

It would be nice to have also the CustomBalloonClosed event on TaskbarIcon (similar to the TrayBalloonTipClosed) - which would be triggered after a custom balloon was closed (both explicitly by calling CloseBalloon() or implicitly by timeout).

My use case: I want to show multiple custom successive balloons (one after another), so I'm storing these balloons in a queue. But, I need to be notified when the previous balloon was closed, so I can move to the next balloon in my queue and show it.

(I'm using TaskbarIcon.CustomBalloon.Closed as a workaround, but this approach is a bit complicated because the Popup instance in the CustomBalloon property is replaced when calling the ShowCustomBalloon, so I have to register handlers to the Popup.Closed event for every new CustomBalloon)