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

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.