Fix reuse of list box header widgets
gnome-software reuses a header widget in its GsUpdatesSection list box. If the order of header row update calls happens in the wrong way, that widget ends up unparented but still in the list box’s forall iteration. See the commit messages for more details.
If this is accepted, can it please be backported to gtk-3-24? It applies cleanly there. gnome-software uses GTK 3.