flip systray in ltr layouts

Summary:
in ltr layouts, put the systray expander arrow at the left instead of
at the right

Test Plan: ran plasma with horiz and vertical panels both in normal modes and --reverse

Reviewers: davidedmundson, Plasma

Reviewed By: davidedmundson, Plasma

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3461