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

Summary:
{F7401179}
The old rectangle was completely white, even in dark themes and wasn't completely consistent.

Test Plan: Open the {nav 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