Improve Statement/Claim GUID handling

[JeroenDeDauw]

Right now the way Claim GUIDs are handled is distinctly sub-optimal.

One cannot add a Statement to an Item if the Claim of the Statement does not have a GUID. In many cases one does not care about the guid, and might not have one. This happens often in tests, and is very annoying.

[Benestar]

One question we have to answer is if GUIDs should be assigned to Claims or Statements.

And this even creates another question: Will claims be used as "front end objects" and can they stand alone or are they always connected to a statement. For the first, the GUID should be assigned to the claim and for the second, it makes only sense to add a GUID to a statement.

Currently, we only have statements being able to be added to entities and I think this will remain so in future so I'd propose to make the GUID a property of the statement and consider Claim only as a data object without any further context assigned.

[JeroenDeDauw]

I'm inclined to agree with moving the GUID to Statement. If at a future point we want to expose something that has the data of a Claim and also has an identity, then we can always create a new object for that.

[JeroenDeDauw]

@mkroetzsch I know we talked about this before. What was your opinion on having a GUID in Statement or Claim? I suspect it was along the line of what I wrote above, though am not sure.