Remove usage of QWeakPointer for QObject for AbstractThumbnailItem::m_parent
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
Differential Revision: https://phabricator.kde.org/D23071