title: ExtJs clear checkboxes in a tree panel
body: <p>In a extjs app, I have a tree panel that is loading json data from a store. In that information I have a property <code>checked</code> that allows manipulate a checkbox over a row in the tree panel. </p>
  
  <p>How can I do to uncheck graphically the checkbox by listening a button? (Clean all checked boxes)</p>
  
  <p>Here's a fiddle that explain a bit the situation.</p>
  
  <p><a href="https://fiddle.sencha.com/#fiddle/17v3" rel="nofollow">https://fiddle.sencha.com/#fiddle/17v3</a></p>