QPushButton/fusion style: don't ignore QIcon::On icon

QPushButton/fusion style: don't ignore QIcon::On icon

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit/?id=e9ccdf4d84157173
Last-Update: 2021-08-10

The fusion style did ignore the QIcon::On icon because it reset
State_On to avoid the visual shift of a pressed button.
But it's not needed to reset this flag - the shift does not happen
because the fusion style does return 0 as offset for
PM_ButtonShiftHorizontal/PM_ButtonShiftVertical so no shifting will
happen.

Gbp-Pq: Name fusion_checkable_qpushbutton.diff