Let filteringMode be configured on Select fields

While trying to configure the filteringMode of a SelectFieldDefinition, I realized it just wasn't working. IIRC, N2B just didn't consider the filteringMode property, because its getter and setter methods used different types. I'd consider just exposing the enum (or a Magnolia-specific version of it if we don't want to expose the Vaadin enum) rather than a meaningless int.

Since MAGNOLIA-5627, Node2Bean supports converting enums in a case-insensitive fashion (and since MAGNOLIA-2734 in case-sensitive).