Add Kirigami ListSectionHeader component

Summary:
Adds a standard ListSectionHeader component to make ListView section headers look coherent.

Test Plan: Run the example with qmlscene:

Reviewers: Kirigami, VDG, mart, ngraham

Reviewed By: Kirigami, VDG, mart, ngraham

Subscribers: jbbgameich, ognarb, davidre, ngraham, VDG, plasma-devel, Kirigami

Tags: Kirigami

Maniphest Tasks: T10384

Differential Revision: https://phabricator.kde.org/D23049