xfce-base/libxfce4ui 4.19.0: breaking ABI change without SOVERSION change

After upgrading libxfce4ui to 4.19.0, we can no longer start xfce4-terminal:

xfce4-terminal: symbol lookup error: xfce4-terminal: undefined symbol: xfce_gtk_menu_append_seperator

It seems that the change was knowingly introduced in !88 (merged) based on the idea that it was "unused" if it its use was removed from xfce4-terminal. However, perceived "use" or not should not be an excuse to break public ABI without increasing SOVERSION appropriately.