Adding / removing sitelinks

[wetneb]

In WikibaseDataEditor, we currently do not expose methods to add or remove sitelinks from entities.

We have updateTermsStatements which lets us add / remove terms and statements, but not sitelinks.

Adding two more arguments to a method like this (or a variant of it) feels like this is becoming pretty heavy. On the long term we should consider providing dedicated classes to represent planned updates to entities (whose fields should roughly be the current arguments of these methods). This is related to #403.