title: How to uncheck the select all checkbox in Ext.selection.CheckboxModel
body: <p>When running a <code>getSelectionModel().clearSelections()</code> on a grid that uses a Ext.selection.CheckboxModel and all rows was selected using the "select all" check box at the top of the select column, the "select all" check box remains checked.</p>
  
  <p>I did a dirty hack to get around this but I wish to know if there is a cleaner way or if this is a bug. </p>