title: Material UI fixed MenuItem
body: <p>I'm wondering is there possibility to add fixed MenuItem to Menu? I'd like to use one of MenuItems to show header. Also if there's another solutions I'm open to try them too.</p>
  <p>Here's the structure:</p>
  <pre><code>&lt;Menu&gt;
      &lt;MenuItem className={classes.header}&gt;This is header&lt;/MenuItem&gt;
      &lt;MenuItem&gt;Item&lt;/MenuItem&gt;
      &lt;MenuItem&gt;Item&lt;/MenuItem&gt;
      &lt;MenuItem&gt;Item&lt;/MenuItem&gt;
  &lt;/Menu&gt;
  </code></pre>
  <p>I tried to set header MenuItem's position to fixed but it throws whole Menu top of the page.</p>
  <pre><code>header: {
     position: 'fixed',
  },
  </code></pre>
  <p>EDIT: To clear a bit what I'm looking for. GitHub has same kind of menu:</p>
  <p><a href="https://i.stack.imgur.com/TiXFn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TiXFn.png" alt="enter image description here" /></a></p>