[Form] Implemented support for buttons
[Form] Made submit buttons able to convey validation groups
[Form] Changed isset() to array_key_exists() to be consistent with ParameterBag
[Form] Fixed concatenation operator CS (see 7c47e34928c39e4797edc841423237a16588395e)
[Form] The option "validation_groups" can now be set to false to disable validation. This is identical to setting it to an empty array.
[Form] Fixed invalid use of FormException
[Form] Fixed incorrect doc comment
[Form] Simplified Twig code
[Form] Removed deprecated code scheduled for removal in 2.3
[Form] Added leading backslashes to @exceptionMessage doc blocks
[Form] Added upgrade information about setting "validation_groups" => false
merged branch bschussek/issue5383 (PR #6528)

This PR was merged into the master branch.

Discussion
----------

[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.

Commits
-------

faf8d7a [Form] Added upgrade information about setting "validation_groups" => false
d504732 [Form] Added leading backslashes to @exceptionMessage doc blocks
c8afa88 [Form] Removed deprecated code scheduled for removal in 2.3
36ca056 [Form] Simplified Twig code
ce29c70 [Form] Fixed incorrect doc comment
0bc7129 [Form] Fixed invalid use of FormException
600007b [Form] The option "validation_groups" can now be set to false to disable validation. This is identical to setting it to an empty array.
277d6df [Form] Fixed concatenation operator CS (see 7c47e34928c39e4797edc841423237a16588395e)
7b07925 [Form] Changed isset() to array_key_exists() to be consistent with ParameterBag
7b438a8 [Form] Made submit buttons able to convey validation groups
cc2118d [Form] Implemented support for buttons