XB: Add support for rendering ce-elements in SDC slots

Motivation

For twig based rendering of SDCs we have to take care of situations where we have slots which are not twig-based, so we need to be able to jump back and forth between twig/render-API based rendering and CE-conversion.

The first issue adds/added support for embedding in one direction, e.g. only support CE-conversion to embed twig-SDC rendered elements.

Proposed resolution

* Extend slot rendering of SDCs to cater for the case of embedded CE-rendered elements like CE-rendered SDCs or CE-rendered blocks.
* For that, first clarify the component used. drupal-markup cannot contain CE-enabled slots. We'd need some component like "drupal-ce-markup" that has multiple slots (e.g. slot[1-10]) next to the markup.