weston: Port X11 backend to new output handling API

weston: Port X11 backend to new output handling API

This is a complete port of the X11 backend that
uses 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.

Same as before, a single output is created at runtime
using the default configuration or a configuration
parsed from the command line. The output-count
functionality is also preserved, which means more than
one output can be created initially, and more outputs can
be added at runtime using the output API.

v2:

 - Fix wet_configure_windowed_output_from_config() usage.
 - Call x11_output_disable() explicitly from
   x11_output_destroy().

v3:

 - Remove unneeded free().
 - Disallow calling x11_output_configure more than once.
 - Remove unneeded checks for output->name == NULL as that
   has been disallowed.
 - Use weston_compositor_add_pending_output().
 - Bump weston_x11_backend_config version to 2.

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