improve positioning for highlight in systray

Summary:
--the highlight item used in the systray now uses
the panel thickness for correct positioning. In
the past there were cases that the highlight item
wasnt shown at all or was positioned a bit far from
the panel edge

Test Plan:
use the systray and investigate that highlight item
is positioned correctly

Reviewers: #plasma, mart, davidedmundson

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5927