Improve the look of the Application Style KCM UI

Summary:
Apply the KDE HIG, merge the two tabs into one page and make the KCM look better.

Test Plan: Open the Application Style KCM.

Reviewers: Plasma, VDG, rooty, ngraham

Reviewed By: VDG, rooty, ngraham

Subscribers: ngraham, broulik, rooty, abetts, ndavis, VDG, plasma-devel, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19651