Adds a new child item to this item.
        
        :param      item | <XGanttWidgetItem>

def addChild(self, item):
        """
        Adds a new child item to this item.
        
        :param      item | <XGanttWidgetItem>
        """
        super(XGanttWidgetItem, self).addChild(item)
        
        item.sync()