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