Expander: Fix parent map caused input to collapsed

Open Daniel Boles requested to merge dboles/issue2069 into gtk-3-24
…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.

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