Add frame_layout classes to CEs

Hi,

how can I add frame_layout classes to content elements? I do not find anything in the docs and the old style does not work:

```
TCEFORM.tt_content.frame_class.addItems {
greybox = graue Box
bluebox = blaue Box
}
```

Thanks!

Start by tracing the TCEFORM.tt_content.frame_class configuration and the frame_layout setting referenced in the issue. Confirm how content elements currently expose these values and document or implement a working configuration; done means custom greybox and bluebox classes can be selected and applied to content elements.

Written by the indexing model from the issue text.