Qt apps - add to existing text

I have just started Qt widget programming. Part of my first program looks like this

Code:

void MainWindow::numberChanged(){
    int numOne=ui->numberOne->text().toInt();
    int numTwo=ui->numberTwo->text().toInt();
    int addition=numOne+numTwo;
    ui->textEdit->setText(QString::number(addition));
}

This void is triggered by a pushbutton. For the line in bold, I want it to ADD to what is already there instead of replacing each time the button is pressed (like the VB textbox1.text=textbox1.text+variable). Oh, and if I just have to read some manual, point one out please. Preferrably on the net.

Not much of a program in itself, but would be useful to know for larger programs.

Why not use +=?

Qt 4.7: QString Class Reference

(EDIT: I'm assuming textEdit is a QString, but I have to admit I don't know enough Qt to know whether it really is.)

Thanks. textEdit is the name of the textfield being used (multiline instead of just one line). It was the default and I didn't bother changing it as its only a practise program for things like calculators, stack etc.