Draft: Fix typo in xfce_gtk_menu_append_seperator

Replace with xfce_gtk_menu_append_separator

Issue: xfce/libxfce4ui#49 (closed)

Problem: By merging this, terminal would depend on a development reelease. So probably better to wait until 4.18 is out ?

Edit: It has the shortcuts editor .. so I suppuse: yes, it is anyhow time to bump the ui dependency :) .. removing Draft status.