Improve the look of the Activities KCM UI
Summary:
Apply the KDE HIG, port to QQC2 and Kirigami, make the KCM look better and make the KCM show up in search results.
Test Plan: Open the Activities KCM and change some settings. Everything that was working before should still work.
Reviewers: Plasma, VDG, ivan, ngraham, KActivities
Reviewed By: Plasma, VDG, ivan, ngraham, KActivities
Subscribers: KActivities, ivan, ngraham, VDG, plasma-devel, Plasma
Tags: Plasma, KActivities
Differential Revision: https://phabricator.kde.org/D23174