Improve the look of the Lock Screen KCM UI

Summary:
Apply the KDE HIG and make the desktop file consistent.
{F7159938}

Test Plan: Open the Lock Screen KCM.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

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

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22911