A Form that knows when all its RegexTextFields and RegexUIs are
valid: enable the submit button with onChanged or RegexForm.isValidOf.
When one field changes, the others are checked again too, so "repeat
password" follows the password. Fields with optional: true may stay
empty. Plain TextFormFields are not counted; Form.validate() still
checks them.
Knows whether every RegexTextField and RegexUI inside a RegexForm
meets its rules.
Listen to it, or read isValid; pass one to RegexForm to use it from
outside the form.
