Add BorderUrgencyHook to XMonad.Hooks.UrgencyHook

BorderUrgencyHook is a new UrgencyHook usable with withUrgencyHook or
withUrgencyHookC; it allows an urgent window to be given a different
border color.  This may not always work as intended, since UrgencyHook
likes to assume that a window being visible is sufficient to disable
urgency notification; but with suppressWhen = Never it may work well
enough.

There is a report that if a new window is created at the wrong time,
the wrong window may be marked urgent somehow.  I seem to once again
be revealing bugs in underlying packages, although a quick examination
of X.H.UrgencyHook doesn't seem to show any way for the wrong window
to be selected.

XMonad/Hooks/UrgencyHook.hs

@@ -59,6 +59,7 @@ module XMonad.Hooks.UrgencyHook (
                                  dzenUrgencyHook,
                                  DzenUrgencyHook(..),
                                  NoUrgencyHook(..),
+                                 BorderUrgencyHook(..),
                                  FocusHook(..),
                                  minutes, seconds,
                                  -- * Stuff for developers:
@@ -83,6 +84,7 @@ import Data.Bits (testBit)
 import Data.List (delete, (\\))
 import Data.Maybe (listToMaybe, maybeToList)
 import qualified Data.Set as S
+import System.IO (hPutStrLn, stderr)
 
 -- $usage
 --
@@ -423,6 +425,30 @@ data FocusHook = FocusHook deriving (Read, Show)
 instance UrgencyHook FocusHook where
     urgencyHook _ _ = focusUrgent
 
+-- | A hook that sets the border color of an urgent window.  The color
+--   will remain until the next time the window gains or loses focus, at
+--   which point the standard border color from the XConfig will be applied.
+--   You may want to use suppressWhen = Never with this:
+--
+--   > withUrgencyHookC BorderUrgencyHook { urgencyBorderColor = "#ff0000" } urgencyConfig { suppressWhen = Never } ...
+--
+--   (This should be @urgentBorderColor@ but that breaks "XMonad.Layout.Decoration".  
+--   @borderColor@ breaks anyone using 'XPConfig' from "XMonad.Prompt".  We need to 
+--   think a bit more about namespacing issues, maybe.)
+data BorderUrgencyHook = BorderUrgencyHook { urgencyBorderColor :: !String }
+                       deriving (Read, Show)
+
+instance UrgencyHook BorderUrgencyHook where
+  urgencyHook BorderUrgencyHook { urgencyBorderColor = cs } w =
+    withDisplay $ \dpy -> io $ do
+      c' <- initColor dpy cs
+      case c' of
+        Just c -> setWindowBorder dpy w c
+        _      -> hPutStrLn stderr $ concat ["Warning: bad urgentBorderColor "
+                                            ,show cs
+                                            ," in BorderUrgencyHook"
+                                            ]
+
 -- | Flashes when a window requests your attention and you can't see it.
 -- Defaults to a duration of five seconds, and no extra args to dzen.
 -- See 'DzenUrgencyHook'.