Provide a qqc2/kirigami-based about page

Summary:
Instead of using the one in xmlgui, create one that integrates properly in Kirigami/Discover.

Depends on D17078 and D17080.

Test Plan: Used it, saw myself in it.

Reviewers: Plasma, Frameworks

Subscribers: vkrause, mart, leinir, ngraham, ltoscano, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17079