xfce_gtk_label_set_a11y_relation
Thunar has a shorthand function to set relation with a label and a widget (thunar_gtk_label_set_a11y_relation()), so assistive technologies can relate two. But other part of XFCE (even including Thunar plugins) do not use this function and use a boilerplate code instead. Since it is used outside Thunar, it would be reasonable to have this on libxfce4ui instead.