Make it possible to use QXmlStreamReader to read a KNS registry file

Summary: It's more efficient, especially in memory

Test Plan: tests pass, discover kns test passes, discover works

Reviewers: Frameworks, leinir

Reviewed By: leinir

Subscribers: leinir, dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6067