get-directory in Dialogs

• (get-directory message parent directory style) -> string or #f
   message = #f : string or #f
   parent = #f : frame% or dialog% object or #f
   directory = #f : string or #f
   style = null : an empty list of symbols

  Obtains a directory pathname from the user via the platform-specific standard (modal) dialog, using parent as the parent window if it is specified.

  If directory is not #f, it is used as the starting directory for the directory selector (otherwise the starting directory is chosen automatically in a platform-specific manner, usually based on the current directory and the user's interactions in previous calls to get-file, put-file, etc.).

  The style argument is provided for future extensions. Currently, style must be the empty list.