[KCM] Port main page to QQC2

Summary:
BUG: 414211
FIXED-IN: 5.18.0

Test Plan: Open Network Connections KCM and use it; no visual or behavioral regressions

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25408