Remove usage of QWeakPointer for QObject for AbstractThumbnailItem::m_parent

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.

Use QPointer instead, which is clean.

Reviewers: #kwin, zzag, romangg

Reviewed By: #kwin, zzag, romangg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23071