X.Prelude: Add findM

Several definitions of this were scattered over a few modules, so just
re-export it from our prelude.

XMonad/Layout/IM.hs

@@ -30,10 +30,11 @@ module XMonad.Layout.IM (
 ) where
 
 import XMonad
-import qualified XMonad.StackSet as S
 import XMonad.Layout.Grid
 import XMonad.Layout.LayoutModifier
+import XMonad.Prelude
 import XMonad.Util.WindowProperties
+import qualified XMonad.StackSet as S
 
 import Control.Arrow (first)
 
@@ -110,11 +111,6 @@ applyIM ratio prop wksp rect = do
             return (first ((w, masterRect) :) wrs)
         Nothing -> runLayout wksp rect
 
--- | Like find, but works with monadic computation instead of pure function.
-findM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
-findM _ [] = return Nothing
-findM f (x:xs) = do { b <- f x; if b then return (Just x) else findM f xs }
-
 -- | This is for compatibility with old configs only and will be removed in future versions!
 data IM a = IM Rational Property deriving (Read, Show)
 instance LayoutClass IM Window where