Fix clipping and use a real ScrollView for the Display Configuration KCM screen overview

Summary:

The old rectangle was completely white, even in dark themes and wasn't completely consistent.

Test Plan: Open the Display Configuration KCM.

Reviewers: Plasma, VDG, ngraham, romangg

Reviewed By: Plasma, VDG, ngraham, romangg

Subscribers: romangg, ngraham, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24129