[Kickoff] Redo tab bar separator's geometry

Summary:
This patch makes Kickoff tab indicators glide (patch D22108) on the tab bar separator in all 4 orientations.

I had to give up on the separator matching the width\height of the tab indicators because it was just never pixel perfect.

Test Plan:
Screenshots made with patch D22108 applied

{F6939067}

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Reviewers: #plasma, #vdg, ndavis, ngraham

Reviewed By: #vdg, ndavis, ngraham

Subscribers: ngraham, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22128