title: Add pseudo element with Symfony Form
body: <p>I have an Entity (let it be User) with text field (let it be "avatar"). </p>
  
  <p>Now i want to create Symfony Form Type for editing User Entity, but i do not want to edit "avatar" field as text. Instead, i want a checkbox "". </p>
  
  <p>If user check on "Clear field avatar", when text filed "avatar" will be cleared. </p>
  
  <p>How to do it? Thanks.</p>