Fix tab bar layout key handling in RTL

Summary:
Current code will, when mirrored, treat all keys like they're the left
key.

BUG: 379894

Test Plan: Ran plasma in RTL

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D6480