Improve the Splash Screen KCM
Summary:
Similar to D18206.
The Splash Screen KCM has some inconsistencies and doesn't always apply the KDE HIG, so this fixes it and makes some other improvements.
Test Plan: Open the Splash Screen KCM
Reviewers: Plasma, VDG, davidedmundson, ngraham
Reviewed By: VDG, ngraham
Subscribers: davidedmundson, ngraham, VDG, plasma-devel, Plasma
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18269