QStyleKitStyle: Add Qt::FramelessWindowHint flag to translucent widgets

QStyleKitStyle sets Qt::WA_TranslucentBackground flag to relevant
widgets (menus, popups) in order to draw rounded corners for them.
Our docs say that Qt::FramelessWindowHint window flag needs to be set
on Windows and macOS for translucency (ie: Qt::WA_TranslucentBackground
flag) to work. So add/remove this flag respectively when setting the
translucency flag to the widget.

Pick-to: 6.12
Change-Id: I8df98158283afe31a267992c13628aac6b4d7039
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>