[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:
{F6780782}

After:
{F6780783}

Reviewers: #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

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