QStyleKitStyle: Factor out autofill and translucency handling in helpers

Move the viewport autoFillBackground and popup WA_TranslucentBackground
handling out of polish()/unpolish() into set/unset helpers, mirroring
set/unsetStyleFont and set/unsetStylePalette. Track the two cases in
separate sets instead of the shared bgFillDisabledWidgets, so unpolish()
no longer has to infer which change to revert.

Pick-to: 6.12
Change-Id: I844b6eb743d8e32090075bfe3d2b581b51ccd626
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>