nautilus-window: fix window maximized notify signal
Nautilus currently does not persist the window maximized state.
This is because we are listening to the property is-maximized for change, which is now maximized in GTK4. https://docs.gtk.org/gtk4/property.Window.maximized.html
To fix this, use notify::maximized instead of notify::is-maximized.
Fixes #2682 (closed)