[plasmawindowed] Pick up KQuickAddons::QtQuickSettings

Summary:
If having the need to use those settings with something else than the
default, one also expects the utils to follow those custom settings.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18625