This seemed like a good idea when the app still used GTK3, but GTK4 already has a built-in video widget. Maintaning this single functionality would be hard and I cannot promise a stable API anyway. The app main and only purpose will be a video player from now on.
GTK since 4.0 has fullscreened property that can be used as both a binding and signal with notify. No need to keep the fullscreen-changed signal as part of the app.
