Remove usage of QWeakPointer for QObject for thumbnails

Summary:
This usage of QWeakPointer has been deprecated since Qt 5.0, since it
leads to really confusing API - usually you must never dereference a
QWeakPointer directly, but always go through QSharedPointer, except in
this one case, where it's permissible.

The thumbnails are only referenced in one place, this change is straight-forward.

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: romangg, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23070