Summary:
KRecursiveFilterProxyModel was only needed to have recursive filtering, which
became available upstream since Qt 5.10.  Also increate min. required Qt version
to 5.10.

Test Plan: The code builds, and recursive filtering still works

Reviewers: #ark, elvisangelaccio, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kde-utils-devel

Tags: #ark

Maniphest Tasks: T12269

Differential Revision: https://phabricator.kde.org/D25631