[2.3] [Form] Support buttons in forms

Bug fix: no
Feature addition: yes
Backwards compatibility break: yes
Symfony2 tests pass: yes
Fixes the following tickets: #5383
Todo: -
License of the code: MIT
Documentation PR: symfony/symfony-docs#2489

The general idea of this PR is to be able to add buttons to forms like so:

```php
$builder->add('clickme', 'submit');
```

You can then check in the controller whether the button was clicked:

```php
if ($form->get('clickme')->isClicked()) {
   // do stuff
}
```

Button-specific validation groups are also supported:

```php
$builder->add('clickme', 'submit', array(
    'validation_groups' => 'OnlyClickMe',
));
```

The validation group will then override the one defined in the form if that button is clicked.

This PR also introduces the disabling of form validation by passing the value `false` in the option `validation_groups`:

```php
$builder->add('clickme', 'submit', array(
    'validation_groups' => false,
));
```

The same can be achieved (already before this PR) by passing an empty array:
```php
$builder->add('clickme', 'submit', array(
    'validation_groups' => array(),
));
```

See the linked documentation for more information.