Actually create the outputmanagement interface

Summary: It's instantiated, but never created. This means that libkscreen's wayland backend is going to hang initializing.

Test Plan: Ran libkscreen against it, works with and doesn't work without patch.

Reviewers: graesslin, Plasma

Reviewed By: graesslin, Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1524