don't override paddings

Summary:
as paddings are used to indicate there should be some space
made up for the scrollbar, don't overwrite them, use negative
margins instead on the items we are sure there should never be
margins regarless the scrollbar is visible or not

Test Plan: better look both with and without scrollbars

Reviewers: apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6610