Summary:
KRecursiveFilterProxyModel is dead. Port to QSortFilterProxyModel.

follow instructions in krecursivefilterproxymodel.h:
"If you can depend on Qt >= 5.10, then just use QSortFilterProxyModel::setRecursiveFilteringEnabled(true), and you don't need to use KRecursiveFilterProxyModel."
WARNING: merging this will break mailcommon build. working on a separate patch for mailcommon. perhaps others projects will also fail. don't know yet.

Reviewers: dvratil, winterz

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D25621