tabbar on proper side when ltr

Summary:
when the layout is mirrored, the anchors are too, we end
up on the tabbar anchored to the wrong side, swap left and right states
when the application is mirrored, to keep the tabbar against the panel

BUG:372720

Test Plan:
tried with plasmashell both normally and with --reverse parameter
in both cases in vertical panels tabbar is on panel side

Reviewers: #plasma

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3433