Support for a Subheadline / Subtitle

Feel free to review the changes. I implemented it so that you can toggle a subtitle per accordion row. The subtitle downcasts to a second

BTW: I just implented this into the CKEditor5 version.
Dear maintainers, would you rather commit this if there was the same implementation for CKEditor4 version as well? Because IMHO its totally fine to have this feature only in cke5 version, what do you think? It seems a little bit unecessary as i would also have to adjust you provided upgrade path as well and perspectively you should upgrade to5 anyway, what do you think?