Symfony - forms with variable number of fields

I want to create form, that serve for adding, editing (and removing when url field is empty) menu items. Problem is that the count of rows/items are variable. (As you can see on the first picture)

Questions:

1)How to write a form that has variable number of fields.

2)How to parse data into the fields at this form.

class GalleryType extends AbstractType {

public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder->add(
    //...some textType, fileType fields,...etc
    );
}

public function configureOptions(OptionsResolver $resolver) {
    $resolver->setDefaults([
        //...
        //some data/data_class option that parse data into the field
    ]);
}

Extra Information:

I am working on own simple content management system with Symfony 3 framework. I want to allow user to add menu item with information like: URL, Title and for instance FA icon, background image,..etc .

-There is always one empty row for adding item and the rest of fields are fulfilled with existing data (menu item/s). When you confirm the form, this row is added into the form (and empty row as well).

-There are few different kind of menu: main menu, slider, side menu, that has diferent type of fields. (you can see it on the second picture)

-Main menu has: title, url and some item can have children items (as sub menu)

-Slider has: title, url, color of title, background image

-Side menu has: title, url and Font Awesome Icon

I have already done form for navigation menu (footer) where is just 2 fields(title and link), but I feel this is not propertly way how to programming it... for illustrative purposes here is how I've done navigation Controller:

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;
use AppBundle\Entity\SMBundle\Navigation;
use AppBundle\Entity\Sett;
use Symfony\Component\Serializer\Serializer;
use Symfony\Component\Serializer\Encoder\JsonEncoder;
use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
use Symfony\Component\Form\Extension\Core\Type\TextType;

class SettingsController extends Controller {

//....

/**
 * @Route("/admin/menu/navigation", name="navigation")
 */
public function navigationAction(Request $request) {
    $set = $this->getDoctrine()->getRepository('AppBundle:Sett')->findOneByName('navigation');
    $navigation = $this->deserializeFromStringToObject('navigation');

    if (!$navigation) {
        $set = new Sett();
        $navigation = new Navigation();
    }

    $form = $this->createFormFromArray($navigation->getLinksArray());
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        $set->setEditedAt(new \DateTime());
        $set->setName('navigation');

        $this->brutalHack($navigation, $form);

        $set->setContent($this->serializeFromObjectToString($navigation));

        // Save
        $this->save($set);
        return $this->redirect($this->generateUrl('navigation'));
    }
    return $this->render("viewSM/menu/navigation.html.twig", array('form' => $form->createView()));
}

 private function deserializeFromStringToObject($name) {
    $object = $this->getDoctrine()->getRepository('AppBundle:Sett')->findOneByName($name);
    if (!$object) {
        return null;
    }
    $serializer = new Serializer(array(new GetSetMethodNormalizer()), array('json' => new JsonEncoder()));
    return $serializer->deserialize($object->getContent(), 'AppBundle\\Entity\\SMBundle\\' . ucfirst($name), 'json');
}

private function serializeFromObjectToString($object) {
    $serializer = new Serializer(array(new GetSetMethodNormalizer()), array('json' => new JsonEncoder()));
    return $serializer->serialize($object, 'json');
}

   private function createFormFromArray(array $collection) {
    $i = 0;
    $formBuilder = $this->createFormBuilder();

    foreach ($collection as $key => $value) {
        $formBuilder
                ->add('url' . $i, TextType::class, ['label' => 'URL ', 'data' => '' . $key, 'attr' => ['class' => 'form-control']])
                ->add('name' . $i, TextType::class, ['label' => 'Titulek ', 'data' => '' . $value, 'attr' => ['class' => 'form-control']]);
        $i++;
    }
    $formBuilder
            ->add('url' . $i, TextType::class, ['label' => 'URL ', 'attr' => ['class' => 'form-control']])
            ->add('name' . $i, TextType::class, ['label' => 'Titulek ', 'attr' => ['class' => 'form-control']])
            ->add('submit', \Symfony\Component\Form\Extension\Core\Type\SubmitType::class, ['label' => 'Uložit', 'attr' => ['class' => 'btn btn-primary']]);
    $form = $formBuilder->getForm();
    return $form;
}

private function save($set) {
    $em = $this->getDoctrine()->getManager();
    $em->persist($set);
    $em->flush();
}

private function brutalHack($navigation, $form) {
    $nav = array();
    if (count($navigation->getLinksArray()) == 0) {
        $nav[$form['url0']->getData()] = $form['name0']->getData();
    }
    for ($i = 0; $i < count($navigation->getLinksArray()); $i++) {
        $key = $form['url' . $i]->getData();
        $value = $form['name' . $i]->getData();
        if ($key != NULL && $value != NULL) {
            $nav[$key] = $value;
        }
    }
    $key = $form['url' . $i]->getData();
    $value = $form['name' . $i]->getData();
    if ($key != NULL && $value != NULL) {
        $nav[$key] = $value;
    }
    $navigation->setLinksArray($nav);
}
//...
}

Entity:

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 * @ORM\Table(name="sett")
 * @ORM\HasLifecycleCallbacks
 */
class Sett  
{
 /**
 * @ORM\Id
 * @ORM\Column(type="integer")
 * @ORM\GeneratedValue(strategy="AUTO")
 */
private $id;

/**
 * @ORM\Column(name="name", length=255)
 */
private $name;

/**
 * @ORM\Column(name="content", type="json_array")
 */
private $content;  

     /**
 * @ORM\Column(name="edited_at", type="datetime")
 */
private $editedAt;

 /**
 * @ORM\Column(name="created_at", type="datetime")
 */
private $createdAt;

/**
 * @ORM\PrePersist
 */
public function onPrePersist()
{
    $this->createdAt = new \DateTime();
}

/**
 * Get id
 *
 * @return integer
 */
public function getId()
{
    return $this->id;
}

/**
 * Set name
 *
 * @param string $name
 *
 * @return Set
 */
public function setName($name)
{
    $this->name = $name;

    return $this;
}

/**
 * Get name
 *
 * @return string
 */
public function getName()
{
    return $this->name;
}

/**
 * Set content
 *
 * @param array $content
 *
 * @return Set
 */
public function setContent($content)
{
    $this->content = $content;

    return $this;
}

/**
 * Get content
 *
 * @return array
 */
public function getContent()
{
    return $this->content;
}

/**
 * Set editedAt
 *
 * @param \DateTime $editedAt
 *
 * @return Set
 */
public function setEditedAt($editedAt)
{
    $this->editedAt = $editedAt;

    return $this;
}

/**
 * Get editedAt
 *
 * @return \DateTime
 */
public function getEditedAt()
{
    return $this->editedAt;
}

/**
 * Set createdAt
 *
 * @param \DateTime $createdAt
 *
 * @return Set
 */
public function setCreatedAt($createdAt)
{
    $this->createdAt = $createdAt;

    return $this;
}

/**
 * Get createdAt
 *
 * @return \DateTime
 */
public function getCreatedAt()
{
    return $this->createdAt;
}
}

Data class:

class Navigation 
{
    private $linksArray;

public function __construct() {
    $this->linksArray=array();
}

function getLinksArray() {
    return $this->linksArray;
}

function setLinksArray($linksArray) {
    $this->linksArray = $linksArray;
}

function add($key,$value){
    $this->linksArray[$key]=$value;
}


}