Leverage EntityReference::preSave to save IEF entities

Problem/Motivation

Coming from #2721349: Nested inline entities must be saved in "inside-out" order, i noted that we make quite a throughout the code to track iIEF entities in $form_state, to finally save them in \Drupal\inline_entity_form\WidgetSubmit::doSubmit. I suppose all that came from D7.

Otoh, looking into \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem, in D8, it can well do the job for us in preSave:

  /**
   * Determines whether the item holds an unsaved entity.
   *
   * This is notably used for "autocreate" widgets, and more generally to
   * support referencing freshly created entities (they will get saved
   * automatically as the hosting entity gets saved).
   *
   * @return bool
   *   TRUE if the item holds an unsaved entity.
   */
  public function hasNewEntity() {
    return !$this->isEmpty() && $this->target_id === NULL && $this->entity->isNew();
  }

  public function preSave() {
    if ($this->hasNewEntity()) {
      // Save the entity if it has not already been saved by some other code.
      if ($this->entity->isNew()) {
        $this->entity->save();
      }
      // Make sure the parent knows we are updating this property so it can
      // react properly.
      $this->target_id = $this->entity->id();
    }
    if (!$this->isEmpty() && $this->target_id === NULL) {
      $this->target_id = $this->entity->id();
    }
  }

Proposed resolution

Remove our $entity->save() logic and let the field do the job. Benefits:
- Largely reduced complexity on our side.
- Works also for special EntityReferenceItem implementations

Update: This works in principle, but only for new entities.

Remaining tasks

- Figure out how to fix the above.

User interface changes

API changes

Data model changes