New module: XMonad.Prelude

This is a convenience module in order to have less import noise.  It
re-exports the following:

  a) Commonly used modules in full (Data.Foldable, Data.Applicative, and
     so on); though only those that play nicely with each other, so that
     XMonad.Prelude can be imported unqualified without any problems.
     This prevents things like `Prelude.(.)` and `Control.Category.(.)`
     fighting with each other.

  b) Helper functions that don't necessarily fit in any other module;
     e.g., the often used abbreviation `fi = fromIntegral`.

XMonad/Layout/Cross.hs

@@ -19,7 +19,7 @@ module XMonad.Layout.Cross(
 
 import XMonad( Dimension, Rectangle(..), LayoutClass(..), Resize(..), fromMessage )
 import XMonad.StackSet( focus, up, down )
-import Control.Monad( msum )
+import XMonad.Prelude( msum )
 
 -- $usage
 -- You can use this module with the following in your @~\/.xmonad\/xmonad.hs@: