Form validation scope

Fully backward compatible except fixing this bug

```php
$form = new Form();
$form->addText('name')->setRequired();
$form->addSubmit('send')->setValidationScope(FALSE);
// submit by send button
if ($form->isSuccess()) {
}
```

Before it doesn't pass isSuccess, because isSuccess doesn't take into account validation scope.
Validation scope was affecting only onSuccess callback.