compositor-wayland: migrate to head-based output API

Follow the standard pattern used in the headless and x11 backend
migration, but also cater for the two other backend modes: --sprawl or
fullscreen-shell, and --fullscreen.

Stops relying on the implicit weston_output::head.

Unlike other backends, this uses the attach_head hook to do the
required head setup that is not possible to do without an output, but
must be done before weston_output_enable(). This also requires the
detach_head hook for the one case where the user attaches a --fullscreen
head and then detaches it without enabling the output.

It is a little awkward to fully initialize heads as late as attach, but
aside from the --sprawl/fullscreen-shell case, there is not really a way
to know the head properties without creating the parent wl_surface and
configuring it.

Heads/outputs created for parent outputs now have distinct names instead
of all being called "wlparent".

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>