I think it would be nice to make the preferences dialogue look like other Xfce style dialogues. It currently is a bit special, and in my opinion consistency is preferrable. One aspect is using an icon bar instead of tabs. Ordinary tabs should be used. Another aspect is the missing Xfce header.
Created attachment 2417 Remove icon bar in favour of tab labels
Moved the dialog to glade (rev. 30109), so this is fixed, thanks for the patch anyway.