Wayland: manage outputs without id mapping

Summary:
Instead of having a separate mapping from KWayland ids to KScreen ids for
output removal connect to the removed signal.

For that create the output device through the Registry instead of binding.

Test Plan: Manually plugged in and removed output in Wayland session.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23004