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.
compositor: remove the weston_config field from weston_compositor

Instead of the central weston_config pointer we now store it in some
module-specific pointers. This way we can remove them one by one.