Set style object on QStyleOptionFrames in QQuickStyleItem

For some controls an explicit QStyleOptionFrame is created. By setting
the styleObject the QStyle can recognize that it's a QQuickItem and
adjust the rendering. This is for example needed in the oxygen style to
get the frame around a menu.

Change-Id: I52d2a8b259477b2ff668827334d5073c6934f23c
Reviewed-by: Martin Klapetek <mklapetek@kde.org>
Reviewed-by: David Edmundson <davidedmundson@kde.org>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>