[RFC] Unify style of new Kirigami.ListSectionHeader and CategoryDrawer

Summary: Before the default style looked quite oxygenish. Now that we have settled on a style with Kirigami.ListSectionHeader component we can use the same style here to improve consistency between QWidget and QML lists that are displayed next to each other.

Test Plan:
Before:
kcategorizedviewtest{F7546011}
Global Shortcuts KCM (without scrollbar)

KRunner KCM (with scrollbar)

After:
kcategorizedviewtest

Global Shortcuts KCM
KRunner KCM

Reviewers: Frameworks, VDG, Konversation, KDE Edu, KDE PIM, KPublicTransport, Amarok, ngraham

Reviewed By: VDG, ngraham

Subscribers: ndavis, ognarb, ngraham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T10384

Differential Revision: https://phabricator.kde.org/D24453