Don't use deprecated QAbstractItemModel::reset()

Summary:
QAbstractItemModel::reset() is deprecated since Qt 5.0. It is strongly
advised to use combination of QAbstractItemModel::beginResetModel() and
QAbstractItemModel::endResetModel() instead.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D22372