Cannot use ckeditor custom styles with the accordions

Problem/Motivation

The ckeditor accordion plugin don't support the custom classes for the ckeditor plugin in source code mode.

If we add a class for the accordion in the source html mode of ckeditor the added class / classes to the accordion are lost on save or ckeditor preview mode.

Proposed Resolution

Add support to add custom class in the source code which then can be upcasted or downcasted by the editor.