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.
{F6688637}

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