integrate an external Filemanager to TinyMCE v5.

[SimJH]

I modified Rich File Manager, it can be used in TinyMCE 5 version.

I'd like you to add the code below to the wiki.

Thank you~!

tinymce.init({
    file_picker_callback: function(callback, value, meta) {
      tinyMCE.activeEditor.windowManager.openUrl({
          url: "",
          title: "File Manager",
          width: window.innerWidth,
          height: window.innerHeight,
          onMessage: function(api, message) {
               callback(message.content, { alt: "File From Rich File Manager" });
          }
      });
    }
});