Fix DownloadPage use of KNS.ItemsModel

Summary: At some point the framework broke compatibility and stopped affecting the engine.engine and wanted the engine directly

Test Plan: I get something in the DownloadPage now

Reviewers: cordlandwehr, leinir, ahiemstra

Reviewed By: leinir

Subscribers: kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D26976