Issues #3371257 and #3367002: Add support for the menu entity.

Closes #3367002 and #3371257

modules/inline_entity_menu_form/src/Plugin/Field/FieldWidget/InlineEntityFormMenu.php

+<?php
+
+namespace Drupal\inline_entity_menu_form\Plugin\Field\FieldWidget;
+
+
+use Drupal\Component\Serialization\Json;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+use Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormComplex;
+
+/**
+ * Menu entity inline widget.
+ *
+ * @FieldWidget(
+ *   id = "inline_entity_form_menu",
+ *   label = @Translation("Inline entity form - Menu"),
+ *   field_types = {
+ *     "entity_reference",
+ *     "entity_reference_revisions",
+ *   },
+ *   multiple_values = true
+ * )
+ */
+class InlineEntityFormMenu extends InlineEntityFormComplex {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function settingsForm(array $form, FormStateInterface $form_state) {
+    $element = parent::settingsForm($form, $form_state);
+
+    $element['form_mode'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Form mode'),
+      '#default_value' => 'edit',
+      '#options' => ['edit' => 'Edit'],
+      '#required' => TRUE,
+    ];
+    return $element;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getInlineEntityForm($operation, $bundle, $langcode, $delta, array $parents, EntityInterface $entity = NULL) {
+    $element = parent::getInlineEntityForm($operation, $bundle, $langcode, $delta, $parents, $entity);
+    $element['#save_entity'] = TRUE;
+    return $element;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function isApplicable(FieldDefinitionInterface $field_definition) {
+    return $field_definition->getFieldStorageDefinition()->getSetting('target_type') === 'menu';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
+    $element = parent::formElement($items, $delta, $element, $form, $form_state);
+
+    $entities = $form_state->get([
+      'inline_entity_form',
+      $this->getIefId(),
+      'entities',
+    ]);
+
+    foreach ($entities as $key => $entity) {
+      $row = &$element['entities'][$key];
+      $title = $this->t('Edit Links');
+      $row['actions']['ief_entity_menu'] = [
+        '#type' => 'link',
+        '#title' => $title,
+        '#url' => Url::fromRoute('inline_entity_menu_form.menu.edit_links_form', ['menu' => $entity['entity']->id()]),
+        '#attributes' => [
+          'class' => ['use-ajax', 'btn-primary'],
+          'data-dialog-type' => 'modal',
+          'data-dialog-options' => Json::encode([
+            'width' => 700,
+            'closeOnEscape' => TRUE,
+            'title' => $title,
+            'autoResize' => TRUE,
+          ]),
+        ],
+      ];
+    }
+
+    return $element;
+  }
+}