title: 1:m and m:m relations: give some information about the associated elements in the backend list
body: <p>I have created a module using the sfGuardUser model. I'd like to show
  in the list of the backend of that module, the groups that a user
  belongs to.</p>
  
  <p>I know the backend is not initially prepared for that, but is there
  any plugin or any other way to do "easily"?</p>
  
  <p>sf 1.4/propel 1.6</p>
  
  <p>Javier </p>