compositor-wayland: move configuration parsing to weston main.c

compositor-wayland: move configuration parsing to weston main.c

Move function load_wayland_backend_config,
wayland_backend_config_add_new_output, wayland_backend_config_release,
weston_wayland_output_config_init from compositor-wayland.c to main.c.
Not a single character is changed in those functions.

Create a glue function load_wayland_backend to use moved functions and
following the new load_*_backend style.

Signed-off-by: Benoit Gschwind <gschwind@gnu-log.net>
Acked-by: Quentin Glidic <sardemff7+git@sardemff7.net>
[Pekka: removed left-over entry from load_backend().]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>