Expands this group item to the inputed state.
        
        :param      state | <bool>

def setExpanded(self, state):
        """
        Expands this group item to the inputed state.
        
        :param      state | <bool>
        """
        if state == self._expanded:
            return
        
        self._expanded = state
        
        # update the button
        btn = self.widget()
        if btn:
            if state:
                ico = 'img/treeview/triangle_down.png'
            else:
                ico = 'img/treeview/triangle_right.png'
            btn.setIcon(QtGui.QIcon(resources.find(ico)))
        
        # update the children
        for child in self.children():
            child.setHidden(not state)