title: JQuery Dropdown checklist uncheck checkbox if there are a certain number of checkboxes already checked
body: <p>I'm working with JQuery Dropdown checklist. I'd like when I'vee checked a certain number of options (three for example) if I check a fourth one it doesn't check. </p>
  
  <p>I've seen there is: <code>onItemClick: function(checkbox) {}</code>. Inside this function is there any way of knowing the number of checked elements ? I haven't got it.</p>
  
  <p>I've also tried with change event of the underlying html select but no luck (I get something but not totally the functionality I need).</p>