Insert new element.

        Usage:
            window.add(widget, **{
                'prop1': val,
                'prop2': val2
            })

def add(self, widget, *args, **kwargs):
        """Insert new element.

        Usage:
            window.add(widget, **{
                'prop1': val,
                'prop2': val2
            })

        """
        ins_widget = widget(*args, **kwargs)
        self.__iadd__(ins_widget)
        return ins_widget