This method gets called by QRangeControl whenever the range has changed. It adjusts the default validator and updates the display. Handles resize events for the spin box. Reimplemented from QWidget. The prefix is prepended to the start of the displayed value.
Typical use is to indicate the unit of measurement to the user.
To turn off the prefix display, call this function with 0 or an empty string as parameter. The default is no prefix. Sets the special-value text to text. Your code must then interpret the spin box value of -1 as the user requesting automatic margin width. Neither prefix nor suffix, if set, are added to the special-value text when displayed.