[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