Small updates to libgui and settings manager dialog

[Olivier Fourdan]

> which creates a dialog header looking like this:
> 
> http://members.home.nl/jbhuijsmans/screenshots/20030108.jpg
> 

Or like this :

http://www.xfce.org/archive/xfwm4-mcs.png

(yes, xfwm4 has its own mcs plugin, and it works, but I'm too tired now
to put that on CVS :-P )

Good night,
-- 
Olivier Fourdan <fourdan at xfce.org>
http://www.xfce.org