Deskbar panel mode

[Andrzej]

The xfce panel with the deskbar mode (and multi-row layout) is now 
fairly functional (I'm using it in my daily work) and ready for testing.

So far I'm sticking to the original design with 3 new properties:
- nrows (guint=1) in the panel
- deskbar-mode (gboolean=FALSE) in the panel
- small (gboolean=FALSE) in the plugin base class.

I know there are some objections to the last one, I'm ready and willing 
to add some changes once we have a good alternative. So far, the "small" 
property did a rather good job so I'd be most happy keeping it in the 
code as is.

There are also many plugin changes to discuss on the case by case basis. 
It'd be extremely useful if you could please get the code and try it for 
a while.

The code is at git://github.com/andrzej-r/xfce4-panel.git

Thanks,

Andrzej