Get rid of deprecated QModelIndex::child()

Summary:
QModelIndex::child() is obsolete

https://doc.qt.io/qt-5.13/qmodelindex-obsolete.html

This patch replaces it with QAbstractItemModel::index as recommended.

Test Plan:

1. Compile and install Ark.
2. Open some archive and try to rename an item in it.

Reviewers: Ark, elvisangelaccio

Reviewed By: Ark, elvisangelaccio

Subscribers: elvisangelaccio, kde-utils-devel

Tags: Ark

Differential Revision: https://phabricator.kde.org/D23254