Allow to Add child item like on core menu items

I'll provide a proper MR as I have the code ready but needs some review

Pipelines were already failing before but fixed own coding standards.

I've just found this issue.
Just for the record so I don't forget, I think we might need in the GroupContentMenuForm::buildOverviewTreeForm some code like:

        // Build a list of operations.
        $operations = [];

        if ($element->depth < $this->menuTree->maxDepth()) {
          $add_link_url = Url::fromRoute(
            'entity.group_content_menu.add_menu_link',
            [
              'group' => $group->id(),
              'group_content_menu' => $this->entity->id(),
              'menu_link_content' => $link->getMetaData()['entity_id'],
              'menu' => $this->entity->id(),
            ],
            ['query' => ['parent' => $link->getPluginId()]],
          );
          $operations = [
            'add-child' => [
              'title' => $this->t('Add child'),
              'weight' => 20,
              'url' => $add_link_url,
            ],
          ];
          uasort($operations, [SortArray::class, 'sortByWeightElement']);
        }

Not sure about it yet, since I am testing it in my local, which could be outdated