[containment] Modernize tweaks page layout and port to QQC2

Summary:
Port the page to QQC2 and modernize the layout, using modern Kirigami APIs

This patch removes the invisible group boxes and headers since it seems
unnecessary to give each checkbox in a two-checkbox layout its own private
title. The labels are clear enough already.

Test Plan:
{F6613694}

Everything still works

Reviewers: #vdg, #plasma, mart

Reviewed By: #vdg, #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18990