listbox: Hide the placeholder completely when not needed

Use gtk_widget_set_visible instead of gtk_widget_set_child_visible to set the placeholder visibility. This fixes the problem of having an extra separator in listboxes with hidden placeholders with styles like the .boxed-list in libadwaita.

Fixes #3375 and #4523.

Also fixes gnome-control-center#2193 (closed).

Not sure if this is the correct approach and if it could break some expected behavior. I'm happy to discuss alternative solutions if that's the case.