SelectField: selected values are **filtered** from value state on ite…

…mset change

This addresses a temporary regression in an earlier commit laying groundwork for engineDOMAdapter integration. The nature of the change is best described by #57

Some previously failing scenario tests now pass. And some new tests are added to clarify the change in behavior.

**NOTE:** this change is not intended to press for a decision on #57. It is primarily meant to ensure that, to the best of my knowledge, this refactor does not have any known regressions (excepting some edge cases discussed in previous commit notes). Secondarily, it’s meant to leave some better breadcrumbs for us to make an informed decision on that issue when we do prioritize it.