Deal with help in symfony forms

[odolbeau]

Add a new extractor to deal with the help attribue available since sf4.1.

Fix php-translation/symfony-bundle#234

[Nyholm]

Thank you. This looks good. I will do a more thorough review tonight.

[Nyholm]

Could you help me by explaining the idea behind isKnownNode?

[odolbeau]

To be honest, I have no idea! :)

This visitor is obviously heavily inspired by the FormTypePlaceholder visitor.
I moved the exiting code starting here in the FormTrait in order to use it in both visitors.

Anyway, I don't know in which case we can visit twice the same node exactly. :/

[Nyholm]

Ah okey. I see that you just moved that function. Good. But I think you forgot to remove from PlaceholderFormType, right?

I’m not sure what the use case is either. Try to temporarly remove it and see what test fail :) (if you want to of course. I do not suggest to remove it)

[odolbeau]

The method is called from PlaceholderFormType too (see here).

If it's OK for you, I can remove this function call in another PR in order to not merge two different things at the same time?

I could either make a PR right now or after this one, as you wish?

[Nyholm]

Do it in another one. I think I like this PR as it is. I’ll merge it next time I’m on the computer.

[Nyholm]

Thank you for this PR.