Fix System Tray popup interactivity after echanging item visiblity

Summary:
Avoid using `plasmoid.rootItem` in SystemTray's ConfigEntries.qml.

BUG: 393630
FIXED-IN: 5.14.6

Reviewers: #plasma, ngraham, broulik, davidedmundson, anthonyfieroni, fvogt

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18805