X.L.WorkspaceDir: Export Chdir

This makes it possible to change the workspace directory by sending a
message, which enables automation/scripting not possible with
`changeDir` which displays a prompt.

Example of such usage:
6ea6c52aac/.xmonad/xmonad.hs (L183-L189)

XMonad/Layout/WorkspaceDir.hs

@@ -27,6 +27,7 @@ module XMonad.Layout.WorkspaceDir (
                                    workspaceDir,
                                    changeDir,
                                    WorkspaceDir,
+                                   Chdir(Chdir),
                                   ) where
 
 import System.Directory ( setCurrentDirectory, getCurrentDirectory )