[ScrollViewStyle] Evaluate frameVisible property

ScrollArea has a property for en/disabling the frame provided by its style.
Until now the property wasn't evaluated, which besides making it impossible
to deactivate the frame leads to visual artifacts (observed in Simple Menu:
Flashing frame at the bottom of the page list).

In order to not change the current implementations we additionally set the
value in ScrollArea to true.

Reviewed By: mart

Subscribers: broulik, hein, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4473