Fix EntityOrderProxyModel::setOrderConfig signature

Summary:
so that it can be called like this
orderProxy->setOrderConfig(KMKernel::self()->config()->group("FavoriteCollectionsOrder"));

FavoriteCollectionsModel does the same.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8783