Kill window without focusing it first

This patch requires the patch "add killWindow function" in xmonad.
Before this patch, people would experience “workspace flicker” when closing
a window via EWMH that is not on the current workspace, for example when
quitting pidgin via the panel icon.

XMonad/Hooks/EwmhDesktops.hs

@@ -151,8 +151,7 @@ handle ClientMessageEvent {
         else if mt == a_aw then do
                windows $ W.focusWindow w
         else if mt == a_cw then do
-               windows $ W.focusWindow w
-               kill
+               killWindow w
         else trace $ "Unknown ClientMessageEvent " ++ show mt
 handle _ = undefined -- does not happen, as otherwise ewmhDesktopsHook would not match