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.
{F7248461}
{F7248462}
{F7248463}

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