form: allow custom callback for formData changes

Right now the formData is saved in the formule store #107. We should let users pass their own onChange callback to be able to keep and access the formData without having to access or sync the formule store. The formule store should only be used for editor-related data, and not for already created forms to be filled in.

Start by tracing where formData is stored in the formule store and how created forms receive change events. Define the callback behavior and verify that callers can retain and access formData without synchronizing with the store, while editor-related data continues using the store.

Written by the indexing model from the issue text.