Warn on usages of hidden fields in form definitions

[maxpatiiuk]

For a form editor when we eventually have it: If form definition uses fields that are marked as hidden, there should be warnings emitted This is because such fields won't be easily visible in the query builder or work bench

[grantfitzsimmons]

:+1: Would be really useful!

[maxpatiiuk]

Probably should be part of the form editor v2