title: Add custom class to selected text in tinymce V5
body: <p>I'm trying to implement inside tinymce a custom button, that when clicked, would add a class around the selected text.</p>
  
  <p>I've tried this : </p>
  
  <pre><code>editor.ui.registry.addButton('fwnormal', {
      text: 'N',
      class: 'nongras'
  });
  </code></pre>
  
  <p>But it doesn't work, I get an error <code>Could not find valid *strict* value for "onAction"</code></p>
  
  <p>Any idea how can I do that?</p>