relation players in relation

[hkuich]

currently, only entities can be players in a relation - this needs to be extended and tested to relations and attributes.

[hkuich]

Two parts to this:

1. add attributes and relations as players in the first place
2. add listseparator functionality for players in a relation (relevant for attributes and entities, and relations if they have an ID)

[hkuich]

finished implementing listseparators for players - should not matter if they are entities or relations

relations in relations up next

[hkuich]

• update - put attriubutes as players in relation into separate issue... requires a different way of doing migration altogether... awaiting use case

[hkuich]

implementation finished - merge-request coming