Load the keyboard on-demand

Summary: No need to load the keyboard view as soon as wayland starts. It's not a simple UI and it often will be unused.

Test Plan: played with the keyboard, still worked

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22430