Read file name, prompting with PROMPT in directory DIR.
Use a file selection dialog. Select DEFAULT-FILENAME in the dialog's file
selection box, if specified. If MUSTMATCH is non-nil, the returned file
or directory must exist.
This function is only defined on NS, MS Windows, and X Windows with the
Motif or Gtk toolkits. With the Motif toolkit, ONLY-DIR-P is ignored.
Otherwise, if ONLY-DIR-P is non-nil, the user can only select directories.
(fn PROMPT DIR DEFAULT_FILENAME MUSTMATCH ONLY_DIR_P)