PAnel changes

[Olivier]

Jasper,

I've dropped xinerama support from libxfcegui4 all together.

The panel was still using this impl, so I've changed it to use GdkScreen
instead.

I've also changed the code so that the existing struts are ignored. This
is because I think we said the panel should place itself on screen
edges. Therefore it should ignore other struts (like GNOME panel does).
I think this is more consistent with what the taskbar does now, and
somehow less confusing for the user (not to mention it makes the code a
bit simpler)

I hope you won't mind for these changes to your code.

Cheers,
-- 
 - Olivier Fourdan - fourdan at xfce.org - http://www.xfce.org -