Add two more filter methods to StatementList

We have a `BestStatementsFinder` class. Shouldn't this be reused here? If not, should we rename the class to `BestStatementsByPropertyFinder`, if that's what it does? (CC @Benestar)

Also personally I prefer Bene's implementation with one loop instead of two (see `BestStatementsFinder::getBestStatementsForProperty`). But I would merge this implementation here as it is and do an actual benchmark later, if needed.