[platforms/wayland] Create output devices

Summary:
Since all Wayland session backends now use the same structure of
AbstractWaylandOutput we can create output devices like in the DRM backend.

First let us do this for Wayland nested sessions.

Test Plan: Manually with output-count 1 and 2. Outputs are correctly shown in KScreen.

Reviewers: KWin

Subscribers: zzag, kwin

Tags: KWin

Maniphest Tasks: T11140, T11459

Differential Revision: https://phabricator.kde.org/D23473