Snippet container definition

I would appreciate if I can define snippet block in form definition not only in template because if I need to invalidate only captcha field (TextBase), whole form needs to be defined in template.

I think it hasn't to be as much difficult. It could be one new container type and condition in renderer. If the container type is snippet, render snippet block with given name.

Something like this:

$snippetContainer = $form->addSnippetContainer('captcha');

$snippetContainer->addCaptcha('captcha');