title: Adding another EntityType in a formbuilder in Symfony4
body: <p>is it possible in the Form Builder to add an Entity that is not related to the object where we build the form ? </p>
  
  <p>I try to explain</p>
  
  <p>I've got Article, Category, SuperCategory.
  I want my users to first choose the SuperCategory, then the Category, then the Article.</p>
  
  <p>In the form builder I'd like to add the SuperCategory to the ArticleType, but the SuperCategory is a ManyToOne relation in Category which is a ManyToOne relation in Article.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>$builder-&gt;add('SuperCategory', EntityType::class, [
      'class' =&gt; SuperCategory::class,
      'choice_label' =&gt; 'Title'
  ]);
  </code></pre>
  
  <p>As expected, It doesn't work because the Article class doesn't know about SuperCategory.</p>
  
  <p>Still, I need to show these SuperCategory to make a "FormFlow". Is it possible?</p>
  
  <p>Here's the error (as expected, nothing surprising) :</p>
  
  <pre><code>"Neither the property "SuperCategory" nor one of the methods 
  "getSuperCategory()", "superCategory()", "isSuperCategory()", 
  "hasSuperCategory()", "__get()" exist and have public access in class 
  "App\Entity\Article".
  </code></pre>