People expect to be able to call gtk_widget_show_all on the dialog
to make action widgets visible, as seen e.g. in
https://bugzilla.gnome.org/show_bug.cgi?id=733431
To keep this working, we can't always set no-show-all on the
action_box. Only set it when the action_area is not used and empty.
The real reason for the problem that the just reverted change
was supposed to address is that testfilechooser uses show_all
to tease out places where composite dialogs don't properly
protect their internals against unintended showing. Well,
lets do that, then.
