title: add checkbox to qtreewidgetitem
body: <p>i want to add a check box to my <code>qtreewigetitem</code>, i tried this code to <code>setflag</code>, then i add item is selectable for sake of maybe this will solve my problem but nothing happened, would you please help me how can i add <code>check box</code> to my item? 
  thank you in advance</p>
  
  <pre><code>m_eventList-&gt;addTopLevelItem(new QTreeWidgetItem);
         QTreeWidgetItem *item = m_eventList-&gt;topLevelItem(m_eventList-&gt;topLevelItemCount()-1)
  
      item-&gt;setFlags(item-&gt;flags() | Qt::ItemIsUserCheckable |Qt::ItemIsSelectable);
  </code></pre>