Don't assign a QtObject to a model
Summary:
It's a weird thing to do, and that's causing us to hit
https://bugreports.qt.io/browse/QTBUG-75900 when it tries to lookup a
property (icon) that we don't have.
BUG: 407220
Test Plan: Opened sddm-greeter --test-mode --theme clicked button
Reviewers: Plasma, fvogt
Reviewed By: fvogt
Subscribers: fvogt, apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D21274