Introduce a hint to QPlatformTheme to control the behavior.

Task-number: QTBUG-35231

Change-Id: Ia28e153a8dd3f1931321a222d8906ca87166ed62
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>