title: CKEditor 5 - writer.setAttribute('title', ...) on img element doesn't work
body: <p>I am creating a plugin for CKEditor 5, and I can't figure out how to set the <code>title</code> attribute on an <code>&lt;img&gt;</code> element using <code>writer.setAttribute('title', ...)</code>.</p>
  <p>I have tried stuff like <code>schema.extend</code> but to no avail.
  The thing is, code works flawlessly when operating on the <code>alt</code> attribute.</p>
  <p>Am I missing something?</p>
  <p>My plugin code:</p>
  <pre><code>const ButtonView = require('@ckeditor/ckeditor5-ui/src/button/buttonview').default;
  const imageIcon = require('@ckeditor/ckeditor5-core/theme/icons/low-vision.svg').default;
  
  export default class ImageTextTitle extends Plugin {
  
      init() {
          const editor = this.editor;
  
          editor.ui.componentFactory.add('imageTextTitle', locale =&gt; {
              const view = new ButtonView(locale);
  
              view.set({
                  label: 'Insert image title',
                  icon: imageIcon,
                  tooltip: true
              });
  
              view.on('execute', () =&gt; {
                  const newTitle = prompt('New image title');
                  const selection = editor.model.document.selection;
                  const imageElement = selection.getSelectedElement();
  
                  if (newTitle !== null) {
                      editor.model.change(writer =&gt; {
                          writer.setAttribute('title', newTitle, imageElement);
                      });
                  }
              });
  
              return view;
          });
      }
  }
  </code></pre>