Fix setting the title for the wxDirDialog on OSX

OSX 10.11+ doesn't actually display the title, so update documentation
to reference SetMessage instead. For pre-10.11 override the SetTitle
method to set the title of the NSOpenPanel instead of the window.

Also change so the directory is not updated unless it is provided.

Closes #15143.

interface/wx/dirdlg.h

@@ -71,6 +71,11 @@ const char wxDirDialogNameStr[] = "wxDirCtrl";
     @endcode
     instead of just using @c wxDD_DIR_MUST_EXIST style alone.
 
+    @remarks MacOS 10.11+ does not display a title bar on the dialog. Use SetMessage()
+             to change the string displayed to the user at the top of the dialog after creation.
+             The SetTitle() method is provided for compatibility with pre-10.11 MacOS versions
+             that do still support displaying the title bar.
+
     @library{wxcore}
     @category{cmndlg}