Possible fix for KDecoration crash in systemsettings

Summary:
I could never reproduce the crash, but we know from gdb that it's from
the decorationSettings object

We are setting the same QObject instance as a context property in
multiple contexts at once. This is already slightly odd especially from the POV of
Qt's internal property cache.

Given we want one object to be exposed to all contexts, we can expose it
to the parent context only once and achieve the same result in a simpler
way.

BUG: 411166

Test Plan:
Verified opening and closing system settings still worked for me.
I could never reproduce the original crash.

Reviewers: #kwin, ngraham

Reviewed By: ngraham

Subscribers: ngraham, apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25913