title: Disable drag'n'drop from a level to another one
body: <p>I've a treeview formed from <a href="/questions/tagged/jstree" class="post-tag" title="show questions tagged &#39;jstree&#39;" rel="tag">jstree</a> and I'd like to disable drag'n'drop node from a level to another. Let's take an example:</p>
  
  <ul>
  <li>Branch 1
  
  <ul>
  <li>leaf 1</li>
  <li>leaf 2</li>
  <li>leaf 3</li>
  </ul></li>
  <li>Branch 2
  
  <ul>
  <li>leaf 4</li>
  <li>leaf 5</li>
  </ul></li>
  <li>Branch 3
  
  <ul>
  <li>leaf 6</li>
  <li>leaf 7</li>
  </ul></li>
  <li>Branch 4
  
  <ul>
  <li>leaf 8</li>
  </ul></li>
  </ul>
  
  <hr>
  
  <p>I want the user to be able to move branch/re-order leafs &amp; re-order branches but not promote a <code>leaf</code> as a <code>branch</code> or demote a <code>branch</code> as a <code>leaf</code>.</p>
  
  <p>I've started to take a look at the file <code>jstree.dnd.js</code> to change the comportment but it's out of my league unfortunately.</p>
  
  <p>How can I do this?</p>