[TabBar] Remove exterior margins

Summary: This allows us to glide active tabbar indicators on lines, which we cannot do now in Kickoff due to this limitation in the control.

Test Plan:
Before:

After:

Reviewers: VDG, Plasma, ngraham, ndavis

Reviewed By: VDG, ngraham, ndavis

Subscribers: ndavis, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22108