Improve the look of the GTK Application Style KCM UI

Summary:
Apply the KDE HIG, move the KCM below the Application Style KCM, update strings, fix inconsistencies and make the KCM look better.
{F6696741}

Test Plan: Open the GTK Application Style KCM.

Reviewers: #plasma, #vdg, apol, ngraham

Reviewed By: #vdg, apol, ngraham

Subscribers: ngraham, apol, #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19790