FR: Add addGroup() method to form container

[janvalentik]

This code

$form->addGroup('Group 1');
$container1 = $form->addContainer('container1');
$container1->addText('foo', 'bar');
$form->addGroup('Group 2');
$container2 = $form->addContainer('container2');
$container2_1 = $nak->addContainer('contaniner2_1');
$container2_1->addText('foo', 'bar');
$form->addGroup('Group 3');
$container2_2 = $nak->addContainer('container2_2');
$container2_2->addText('foo', 'bar');

render: