weston: Port Wayland backend to new output handling API

weston: Port Wayland backend to new output handling API

This is a complete port of the Wayland backend that
uses the recently added output handling API for output
configuration.

- Output can be configured at runtime by passing the
  necessary configuration parameters, which can be
  filled in manually, obtained from the configuration
  file or obtained from the command line using
  previously added functionality. It is required that
  the scale and transform values are set using the
  previously added functionality.

- Output can be created at runtime using the output
  API. The output creation only creates a pending
  output, which needs to be configured the same way as
  mentioned above.

However, the backend can behave both as windowed backend
and as a backend that issues "hotplug" events, when
running under fullscreen shell or with --sprawl command
line option. The first case was covered by reusing
previously added functionality. The second case required
another API to be introduced and implemented into both
the backend and compositor for handling output setup.

After everything has been set, output needs to be
enabled manually using weston_output_enable().

v2:

 - Fix wet_configure_windowed_output_from_config() usage.
 - Call wayland_output_disable() explicitly from
   wayland_output_destroy().

v3:

 - Get rid of weston_wayland_output_api and rework output
   creation and configuration in case wayland backend is
   started with --sprawl or on fullscreen-shell.
 - Remove unneeded free().
 - Disallow calling wayland_output_configure more than once.
 - Remove unneeded checks for output->name == NULL as that
   has been disallowed.
 - Use weston_compositor_add_pending_output().

v4:

 - Drop unused fields from weston_wayland_backend_config
   and bump WESTON_WAYLAND_BACKEND_CONFIG_VERSION to 2.
 - Move output creation to backend itself when
   --fullscreen is used.
 - Prevent possible duplicated output names by assigning
   a different name to outputs created without any
   configuration specified.

Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>