compositor: introduce struct weston_backend

This is a preliminary change for libweston, with no functional modifications.
Separate the backends and the core weston_compositor struct, by creating
the weston_compositor in the main(), and having the various backends extend
the weston_backend struct, an instance of which is returned by the backend
entry point.
don't use weston_config in the backends code

This is to make them usable by future libweston users, since they will
not use weston_config.
weston_config is still used in the backends entry points, which will
later on live in their own files.