[Desktop Theme KCM] "Normalize" margin around background in previews

Summary:
Instead of a hard-coded hack for a single theme ("Air") to deal with themes
with large margins, having a generic hack based on heuristics makes
non-default themes look better in the preview.
Not using "widgets/panel-background" for "Air" also results in a better
preview for Air.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19748