Provide means to find original window by its decoration.

In order to enable user to write custom `decorationMouseFocusHook' and
`decorationMouseDragHook' hooks we need to provide him with means to
lookup original window by its decoration.

Module Decoration has internal function `lookFor' for exactly the same
purpose. I exported it under a slightly different name and without
exposing internals of DecorationState.

XMonad/Layout/Decoration.hs

@@ -26,6 +26,7 @@ module XMonad.Layout.Decoration
     , Shrinker (..), DefaultShrinker
     , shrinkText, CustomShrink ( CustomShrink ), shrinkWhile
     , isInStack, isVisible, isInvisible, isWithin, fi
+    , findWindowByDecoration
     , module XMonad.Layout.LayoutModifier
     ) where
 
@@ -307,6 +308,9 @@ lookFor w ((wr,(Just dw,dr)):dwrs) | w == dw = Just (wr,(dw,dr))
 lookFor w ((_, (Nothing, _)):dwrs) = lookFor w dwrs
 lookFor _ [] = Nothing
 
+findWindowByDecoration :: Window -> DecorationState -> Maybe (OrigWin,(Window,Maybe Rectangle))
+findWindowByDecoration w ds = lookFor w (decos ds)
+
 -- | Initialize the 'DecorationState' by initializing the font
 -- structure and by creating the needed decorations.
 initState :: DecorationStyle ds Window => Theme -> ds Window -> Rectangle