Toggles the index's check state.
        
        :param      modelIndex | <QModelIndex>

def toggleModelIndex(self, modelIndex):
        """
        Toggles the index's check state.
        
        :param      modelIndex | <QModelIndex>
        """
        if not self.isCheckable():
            return
            
        item = self.model().item(modelIndex.row())
        if item.checkState() == Qt.Checked:
            state = Qt.Unchecked
        else:
            state = Qt.Checked
        
        item.setCheckState(state)