Make the Notifications KCM category headers look consistent with other Kirigami lists

Summary:

Test Plan: Open the Notifications KCM and press the Configure... button.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, plasma-devel, VDG, Plasma

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22977