Markers in the document fragment

[pjasiun]

Because we have different API to modify the document and different API to modify document fragment (see https://github.com/ckeditor/ckeditor5-engine/issues/858) markers on the document fragment does not work well. It is a problem when you want to handle for instance markers copy and paste.

We should rethink how markers on the document fragment could work after we unify the API to modify the tree model.

[pjasiun]

Since https://github.com/ckeditor/ckeditor5-engine/issues/858 is closed now, we can remove the special mechanism to handle markers in document fragments/upcast, including markers property in the document fragment class:
https://github.com/ckeditor/ckeditor5-engine/blob/2462d27dee6029161853c7f0bbae8953049bd768/src/model/documentfragment.js#L39

[Reinmar]

Specific issue for the copy/cut/drag/drop of markers: #10331