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