KRun QML proxy: port away from deprecated KRun API

Summary:
We want runApplication() instead of runService(), and there's
already a method doing that in this class --- confusingly named
openService()

Test Plan: Builds. Not tested.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: Frameworks

Differential Revision: https://phabricator.kde.org/D22587