QSpinBox: don't use hard-coded constant for the up/down button width

The QTBUG-67126 was reported to prevent a QDoubleSpinBox sizeHint from becoming too small.

However, the fix uses a hard-coded constant of 20 pixels for the width of a spin up/down button (in QCommonStyle.cpp).

const int buttonWidth = (vopt->subControls & (QStyle::SC_SpinBoxUp | QStyle::SC_SpinBoxDown)) != 0 ? 20 : 0;

This causes the area reserved for the spin up/down button to become too narrow on a 4K monitor. It seems that it should get the width of a spin up/down button from the (proxy) style.

Note that this bug was not present in Qt 5.6.3.