Make source conref recognizable in preview and editor

Conreffed elements in the content are easy to recognize. On the one hand, because such objects have a different color and on the other hand because of the conref attribute that contains a value.

Unfortunately, it is not possible to see if the content is being referenced. By this, I mean that content does not show or that it is being used as conref. This is a handicap in the use of conref.