Expander: Fix parent map caused input to collapsed

…child, because we were not calling update_child_mapped(), so the hidden
child still floated in the aether silently waiting to wrongly take input

Close https://gitlab.gnome.org/GNOME/gtk/-/issues/2069

main seems very different in all these areas, and looks like it probably does this right, so I'm presuming this bug is not present there.