title: Forcing an item to remain in place in a jQuery UI Sortable list
body: <p>I've set up a jQuery sortable list but I need to be able to keep some items in the sortable 'fixed' in place and for the other items to sort around them. I thought that there would be built in methods to achieve this, alas this is not the case.</p>
  
  <p>I'm able to set up the list and not include the items in question:</p>
  
  <pre><code>&lt;ul id="fruit"&gt;
      &lt;li class="fixed"&gt;apples&lt;/li&gt;
      &lt;li&gt;oranges&lt;/li&gt;
      &lt;li&gt;bananas&lt;/li&gt;
      &lt;li&gt;pineapples&lt;/li&gt;
      &lt;li&gt;grapes&lt;/li&gt;
      &lt;li class="fixed"&gt;pears&lt;/li&gt;
      &lt;li&gt;mango&lt;/li&gt;
  &lt;/ul&gt;
  
  &lt;script type="text/javascript"&gt;
      $('#fruit').sortable({items: "li:not('.fixed')"})
  &lt;/script&gt;
  </code></pre>
  
  <p>This stops me from drag/dropping these items but they still move if the items around them are sorted. There may be a way to do this using the many callbacks that this method has but I've been unable to work this out.</p>
  
  <p>Maybe this method would be a good addition to the UI Sortable options?</p>