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