[Kickoff] Make the tabbar separator width consistent with tab selection line

Summary: This minor patch shortens the tabbar separator so that it is not longer than the width of leftmost and rightmost tab selection line.

Test Plan:
Before:

After:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20676