[xmonad] tabbed and the way the new decoration framework works

[Andrea Rossato]

a last note: the fact that the decoration windows (with
override_redirect attribute set to True) are in the list returned by
the Decoration modifier, has an interesting side effect: a modifier
placed over Decoration is able to find decorations and knows that the
head of the remainder of the list is the window that decoration has
been created for...

actually I have no idea why someone would place a modifier on top of
Decoration in order to manipulate the returned list, but there are
quite weird guys around...
;)
andrea