Method undo

src/backend/matrix/matrixcommands.h:147–153

			m_private_obj->clearColumn(i);
	}
	void undo() override {
		int last_row = m_private_obj->rowCount() - 1;
		if (last_row < 0)
			return;
		for (int i = 0; i < m_private_obj->columnCount(); i++)
			m_private_obj->setColumnCells(i, 0, last_row, m_backups.at(i));
	}

private:
	MatrixPrivate* m_private_obj;