GTK4: Since GTK4 needs to add the popup menu to the parent widget... We need to remove it after the popup closes, so we trap the "close" signal and try to remove the popup from the parent. Since the "close" signal gets called before the menu's "activate" signal, we have to delay unparenting until after the "activate" signal handler is run. Otherwise the menu objects won't be valid for the "activate" handler, therefore we add an idle callback to unparent the widget when the rest of the processing is done.