Small RTL fixes with the OverlayWidget

Summary:
The detailed information widget isn't mirrored in RTL locales. This fix that.
It also use the Sunken shadow, which is not mirrored for RTL layouts. For now let's just
use Plain.

Reviewers: KDE PIM, mlaurent

Reviewed By: KDE PIM, mlaurent

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D9897