Prefer safe alternatives to getWindowAttributes

Whenever possible, prefer the safe wrappers withWindowAttributes or
safeGetWindowAttributes to getWindowAttributes.

Places where these are not applicable are limited to layouts, where
there is not good "default value" to give back in case these calls fail.
In these cases, we let the exception handling of the layout mechanism
handle it and fall back to the Full layout.

Fixes: https://github.com/xmonad/xmonad-contrib/issues/146

XMonad/Layout/FixedColumn.hs

@@ -23,17 +23,9 @@ module XMonad.Layout.FixedColumn (
         FixedColumn(..)
 ) where
 
-import Graphics.X11.Xlib (Window, rect_width)
-import Graphics.X11.Xlib.Extras ( getWMNormalHints
-                                , getWindowAttributes
-                                , sh_base_size
-                                , sh_resize_inc
-                                , wa_border_width)
-
-import XMonad.Prelude (fromMaybe, msum, (<&>))
-import XMonad.Core (X, LayoutClass(..), fromMessage, io, withDisplay)
-import XMonad.Layout (Resize(..), IncMasterN(..), tile)
-import XMonad.StackSet as W
+import XMonad
+import XMonad.Prelude
+import qualified XMonad.StackSet as W
 
 -- $usage
 -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@:
@@ -82,9 +74,10 @@ instance LayoutClass FixedColumn Window where
 --   columns wide, using @inc@ as a resize increment for windows that
 --   don't have one
 widthCols :: Int -> Int -> Window -> X Int
-widthCols inc n w = withDisplay $ \d -> io $ do
-    sh <- getWMNormalHints d w
-    bw <- fromIntegral . wa_border_width <$> getWindowAttributes d w
+widthCols inc n w = do
+    d  <- asks display
+    bw <- asks $ fi . borderWidth . config
+    sh <- io $ getWMNormalHints d w
     let widthHint f = f sh <&> fromIntegral . fst
         oneCol      = fromMaybe inc $ widthHint sh_resize_inc
         base        = fromMaybe 0 $ widthHint sh_base_size