specialValueText for spin box's maximum value, too

Right now, QAbstractSpinBox allows to set a text to be shown if the spin box's value equals the minimum via the setSpecialValueText function. For a number of applications, a special value for the maximum would be useful, too. For example, think of a spin box to limit the network bandwidth. A potential special value for the minimum would be "off" or "idle", but for the maximum you would like to have "unlimited". Of course, the developer would have to check if the spin box's value equals the maximum (reasonably set) and disable all limits.

As introducing a new special value text function may require renaming the existing one (e.g. setMinimumValueText and setMaximumValueText), I suggest this feature for Qt6.

This suggestion is related to Qt bug 541.