[mailcommon] Port to QSortFilterProxyModel from deprecated KRecursiveFilterProxyModel

Summary:
following https://phabricator.kde.org/D25595

WARNING: don't merge this until D25595 is accepted and merged

Reviewers: dvratil, winterz

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D25596