Improve the look of the Plymouth Boot Splash Screen KCM UI

Summary:
BUG: 398469
BUG: 408573

Port the KCM to the new GriedView KCM design.

Credits go to @broulik, he basically did all the work.

Test Plan: Open the Plymouth Boot Splash Screen KCM.

Reviewers: Plasma, VDG, ngraham, broulik

Reviewed By: Plasma, VDG, ngraham, broulik

Subscribers: davidedmundson, abetts, ngraham, broulik, VDG, plasma-devel, Plasma

Tags: Plasma

Maniphest Tasks: T7270

Differential Revision: https://phabricator.kde.org/D20549