Document expected Item behavior for SceneObject

[Licini]

          currently there is some duplication between the behaviour of the scene itself and its tree attribute. when this behaviour is merged, i assume the guid of the data object will play a role in keeping track of things. in that context, the name of the item is not important. it might even be better to not implicitly reuse the name of the item, but instead make the user assign a name to the scene object explicitly.

in any case, a scene should be serialisable. so the default behaviour in the core library should be that the items implement the data interface.

what downstream users do with this is of course up to them. in that regard, rather than saying that an item should be a data item, perhaps we should define the required behaviour of an item instead, and leave everything up to the downstream implementations...

how does that sound?

Originally posted by @tomvanmele in #1274 (comment)