Use QSlider, set the range to be 0 - 180, set the singleStep property to 1, and connect a slot to the QSlider::valueChanged(). In your slot, convert the integer value to ASCII and send it to your servo controller.

If you don't understand what this means, then I suggest you look at the Qt documentation and the many, many Qt examples and tutorials that came with your Qt distribution and teach yourself the fundamentals of Qt GUI programming.