Fix fields comparison filter ignoring aggregation

Problem 1: The Views/global filter Filter Fields comparison doesn't use the aggregation settings for the selected fields. The fields selection UI pretends it does:

but the resulting query doesn't use COUNT, because the handler's query() doesn't consult aggregation settings (like views_plugin_query_default::compile_fields() does).

Solution: use the same logic to apply aggregation functions to selected fields.

Problem 2: you can't use COUNT in WHERE if the WHERE would influence the COUNT result.
Solution: the condition has to move to a HAVING, which is no problem.

Problem 3: If the condition is grouped (in an OR for instance) with another field, that is NOT in the HAVING, the OR would stretch across WHERE and HAVING, which is impossible.
Solution: Move the entire group to HAVING if any of its conditions requires HAVING.

Yes, problems. I have a patch that fixes problems 1 and 2, but not 3, because that's a very tiny potential problem. It totally messes up the query though. Not fatal, but it won't do what you want.