[xmonad] Decoration rewrite

[wagnerdm at seas.upenn.edu]

Quoting Norbert Zeh <nzeh at cs.dal.ca>:

> The reason why I'm writing about this *before* putting the effort  
> into testing
> all my changes is that I'd like to know how the community feels  
> about a change
> in the interface of the DecorationStyle type class.  As it is, there are only

Is this interface one which is likely to be used in user configs?

That is, after you make the necessary changes to XMC code, do you  
think it's likely users will also have to update their configs in the  
same way? Or are the most public APIs the same?

~d