Port System Settings sidebar to QQC2

Summary:
{F7206388}
Also makes QML imports consistent.

Test Plan: Open the System Settings. Everything should still work.

Reviewers: #plasma, #vdg, ngraham, mart, broulik

Reviewed By: #plasma, #vdg, ngraham, broulik

Subscribers: mart, filipf, ngraham, broulik, #vdg, plasma-devel, #plasma

Tags: #plasma

Maniphest Tasks: T10862

Differential Revision: https://phabricator.kde.org/D22896