[kcmkwin/kwinscreenedges] Improve the look of the Screen Edges and Touch Screen KCM UI

Summary:
Apply the KDE HIG, use a form layout, make the desktop file consistent and make the KCM look better.
{F7160251}
{F7149956}

Test Plan: Open the Screen Edges and Touch Screen KCMs.

Reviewers: #kwin, #plasma, #vdg, zzag, ngraham

Reviewed By: #kwin, #plasma, #vdg, zzag, ngraham

Subscribers: ngraham, zzag, #vdg, #plasma, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22909