Guess parameters for event by type hint

[milo]

• new feature
• BC break? I hope no
• doc PR: will

When you ask for Form values in event by 2nd parameter, you usually do not require form instance at all. For example:

$form->onSuccess[] = function (Form $form, array $value) {
    # I do not need $form here at all
};

This is propose to pass only arguments required by event.

$form->onSuccess[] = function (array $data) {...};
# or
$form->onSuccess[] = function (MyFormData $data) {...};