title: How to add a tick mark to a slider if it cannot inherit QSlider
body: <p>I have a Qt dialog and there is a slider in it, when the dialog is initialized the slider will be set a value. In order to remind the user what is the default value, I want to add a mark to the slider, just draw a line or a triangle above the handle. Here, the slider should be of QSlider type, that means I can't implement a customized control derived from QSlider. Is there any way to realize it ? </p>