Code seems to miss an about dialog, since it calls xfce_panel_plugin_menu_show_about() but does not connect a signal, as posted on the ML by Og Maciel.
Created attachment 1771 Patch by Og Maciel Jannis' respond on the ML: Good point, I should add that dialog. Your patch however seems to be broken (I think it uses undefined macros like CDDRIVE_TITLE and VERSION). But that's an easy fix. I'll look into it the next couple days. Thanks for the pointer - and the patch!
Created attachment 1772 Patch using gtk_show_about_dialog()
Added the about dialog, based on the patch submitted here.
*** Bug 6605 has been marked as a duplicate of this bug. ***