Merge: app.nit: propagate events (such as a button press) to the parents of the control

UI events, such as a button press or a check box toggle, were only sent to the observers of a control, and the control itself. For this reason, every button was watched by the window or some other container.

Propagating the events to the parents (direct and indirect) reaches the window and all of its containers. This allows for more modular code and components, and it removes the need for manually declaring the window as an observer.

Pull-Request: #2199
Reviewed-by: Jean Privat <jean@pryen.org>