[WIP] Add requirements for language fall backs.

I'll try to summarize our discussion from yesterday:

- Have no explicit, generalizeable (to other parts of the data model) marker for inferred, we probably need that later, but can get by for this case without.
- Change deserializer to drop TermFallback silently (I don't like this, although it is at least better than serializing them as Term and silently adding them to the database. Should investigate how hard it is to output an error in the API based on an exception in the deserializer.) At a later point when we have an inferred flag we can start suporting TermFallback deserialization.
- FilterLanguages that has as input an EntityDocument, an AcceptLanguageSet, a Transliterator or something like that and has as output the new EntityDocument will reside in WikibaseDataModel