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-contrib.cabal

@@ -298,6 +298,7 @@ library
                         XMonad.Layout.WindowSwitcherDecoration
                         XMonad.Layout.WorkspaceDir
                         XMonad.Layout.ZoomRow
+                        XMonad.Prelude
                         XMonad.Prompt
                         XMonad.Prompt.AppLauncher
                         XMonad.Prompt.AppendFile