[kcmkwin/kwindesktop] Improve the look of the Virtual Desktops KCM UI

Summary:
Apply the KDE HIG, make the list headers and desktop file consistent.

Test Plan: Open the Virtual Desktops KCM.

Reviewers: KWin, Plasma, VDG, hein, ngraham

Reviewed By: Plasma, VDG, hein, ngraham

Subscribers: broulik, ngraham, VDG, Plasma, kwin, KWin

Tags: KWin

Maniphest Tasks: T10862

Differential Revision: https://phabricator.kde.org/D22933