Updates the items to reflect the current check state system.

def updateCheckState( self ):
        """
        Updates the items to reflect the current check state system.
        """
        checkable = self.isCheckable()
        model     = self.model()
        flags     = Qt.ItemIsSelectable | Qt.ItemIsEnabled
        
        for i in range(self.count()):
            item = model.item(i)
            
            if not (checkable and item.text()):
                item.setCheckable(False)
                item.setFlags(flags)
            
            # only allow checking for items with text
            else:
                item.setCheckable(True)
                item.setFlags(flags | Qt.ItemIsUserCheckable)