Fix crash in Screen Locker KCM on teardown

Summary:
As described in https://bugreports.qt.io/browse/QTBUG-57714 exposing a
QWidget as a context item to QtQuick can crash. Especially as the
engine's context property gets deleted whilst deleting the parent item.

This patch reworks the code so that exposed properties go through a
proxy QObject.

CCBUG: 373628

Test Plan:
Pressing back whilst in the screenlocker KCM used to crash every time,
now it doesn't.

Also changed the wallpaper type, new settings appeared, I went from flat
image to a specific wallpaper, and then locked my screen.

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4535