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/Prompt.hs

@@ -60,6 +60,7 @@ module XMonad.Prompt
     , moveHistory, setSuccess, setDone, setModeDone
     , Direction1D(..)
     , ComplFunction
+    , ComplCaseSensitivity(..)
     -- * X Utilities
     -- $xutils
     , mkUnmanagedWindow
@@ -200,6 +201,8 @@ type ComplFunction = String -> IO [String]
 type XPMode = XPType
 data XPOperationMode = XPSingleMode ComplFunction XPType | XPMultipleModes (W.Stack XPType)
 
+newtype ComplCaseSensitivity = ComplCaseSensitive Bool
+
 instance Show XPType where
     show (XPT p) = showXPrompt p