SQLite test fix -- refs #9057
Fixed #18356 -- Gave the test client signals.template_rendered call a unique dispatch_uid

This prevents the test client context from being lost when the client
is used in a nested fashion.
Fixed #19617 -- Refactored form metaclasses to support more inheritance scenarios.

Thanks apollo13 and mjtamlyn for the reviews.
Fixed #8620 -- Updated the Form metaclass to support excluding fields by shadowing them.