title: Sonata Admin - sonata_type_collection: select from the list of existing entities
body: <p>I'm trying to implement many-to-many relationship between <code>Page</code> and <code>Image</code> entity using <code>PageHasImage</code> bridge entity. In the <code>PageAdmin</code> I added the field like so:</p>
  
  <pre><code>-&gt;add(
        'galleryImages', 
        'sonata_type_collection', 
        array(
              'cascade_validation' =&gt; false,
              'by_reference' =&gt; false,
              'type_options' =&gt; array('delete' =&gt; false)
             ),
        array(
              'edit' =&gt; 'inline',
              'inline' =&gt; 'table',
              'sortable' =&gt; 'position',
              'admin_code' =&gt; 'sonata.admin.page_has_image'
        )
   )
  </code></pre>
  
  <p>which gives me an option to add new <code>Image</code> field, and then either upload new image or select it from the list of uploaded ones. The upload part works as expected, but selecting from the already uploaded images list does nothing. The <code>Select</code> button in <code>Images</code> list has just '#' in its href and doesn't seem to do anything useful. There are also checkboxes next to listed images but no action buttons to make any use of the selection.  </p>
  
  <p>What should I configure to make the actions on the list mean anything?  My guess is there's something missing in the setters of relationsships or in lifecycle event methods but I'd apprecieate some guidance. </p>