This patch implements read access to the outputmanagement interface in
kwin_wayland's drm backend.

- outputdevices are created in DrmOutput, just like the wl_outputs
- wayland_server implements the outputmanagement interface and
- passes the changesets down into the backend

This means that the interface is announced, independently of the DRM
backend, but the actual outputs are currently only there if the DRM
backend is used.

The changes are not applied (passed into the kernel's drm interface
yet). This is obviously work-in-progress, so it's incomplete. Since it
allows us to run kwin[master] with the libkscreen KWayland backend, it's
a significant step allowing testing and further development.

Reviewed-by: Martin Gräßlin