prompt completion: case-sensitivity

Provide a way to perform case-insensitive file / directory completion.
We're using compgen to generate completion candidates, and this is
case-sensitive by default. We can control this by setting the
completion-ignore-case Readline variable prior to invoking compgen. If
we're running a Bash with Readline support, this works as expected.
Otherwise, it has no effect -- completion candidates are still returned,
but compgen generates them in a case-sensitive manner.

To avoid breaking changes, the signatures and behavior of existing
exported functions are unchanged:
  - XMonad.Layout.WorkspaceDir.changeDir
  - XMonad.Prompt.Directory.directoryPrompt
  - XMonad.Prompt.Shell.getShellCompl

New variations of these functions are provided, allowing the caller
to specify the desired case-sensitivity via a ComplCaseSensitivity
argument:
  - XMonad.Layout.WorkspaceDir.changeDir'
  - XMonad.Prompt.Directory.directoryPrompt'
  - XMonad.Prompt.Shell.getShellCompl'

The XMonad.Prompt.Shell exports a couple new functions:
  - compgenDirectories
  - compgenFiles

We make use of this in XMonad.Prompt.Directory to avoid duplicating the
compgen code.

XMonad/Layout/WorkspaceDir.hs

@@ -26,6 +26,7 @@ module XMonad.Layout.WorkspaceDir (
                                    -- $usage
                                    workspaceDir,
                                    changeDir,
+                                   changeDir',
                                    WorkspaceDir,
                                   ) where
 
@@ -33,8 +34,8 @@ import System.Directory ( setCurrentDirectory, getCurrentDirectory )
 import Control.Monad ( when )
 
 import XMonad hiding ( focus )
-import XMonad.Prompt ( XPConfig )
-import XMonad.Prompt.Directory ( directoryPrompt )
+import XMonad.Prompt ( ComplCaseSensitivity (ComplCaseSensitive), XPConfig )
+import XMonad.Prompt.Directory ( directoryPrompt' )
 import XMonad.Layout.LayoutModifier
 import XMonad.StackSet ( tag, currentTag )
 
@@ -87,4 +88,7 @@ scd :: String -> X ()
 scd x = catchIO $ setCurrentDirectory x
 
 changeDir :: XPConfig -> X ()
-changeDir c = directoryPrompt c "Set working directory: " (sendMessage . Chdir)
+changeDir = changeDir' (ComplCaseSensitive True)
+
+changeDir' :: ComplCaseSensitivity -> XPConfig -> X ()
+changeDir' csn c = directoryPrompt' csn c "Set working directory: " (sendMessage . Chdir)