Add two more filter methods to StatementList

This method is replacing an equivalent, though less nice and less efficiently implemented, in WB.git. Using `BestStatementsFinder` was not practical there.

If you just want the best statements, then there is no need to group stuff by property. And if you just want the best statements for a single property, then again there is no need to first create property groups. this method thus ought to be better suited for those use cases then `BestStatementsFinder`.