SpinBox step buttons should respect EtchDisabledText style hint

In WindowsStyle currently the step up/down buttons of a QSpinBox are always drawn with a shadowed arrow when the element is disabled. I think they should take into account the EtchDisabledText style hint, as does e. g. the text on disabled buttons. Reasoning: We need this setting in a simplified style adapted from QWindowsStyle, in lack of some other lo-fi style that's usable on small displays with restricted color space.
A corresponding patch is attached.