This function need not be concerned with prefix or suffix or special-value text, the QSpinBox handles that automatically. This method gets called by QRangeControl whenever the range has changed. It adjusts the default validator and updates the display. 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.
The default is no prefix. 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. To turn off the special-value text display, call this function with an empty string as parameter. The default is no special-value text, i.e. the numeric value is shown as usual.