title: React Admin: Select existing image or upload new
body: <p>usually when working with images in an admin-panel you have the possibility to choose an existing image or to upload a new one. I wonder how this could be achieved with react admin.</p>
  
  <p>So I have two questions:</p>
  
  <ol>
  <li>Is there a way to choose an existing resource or to create a new one "inline"? E.g this is a common pattern with Tags.</li>
  <li>How would I approach the selection of an existing image? Using <code>ReferenceInput</code> with a custom renderer?</li>
  </ol>