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.
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.
compositor: add API to manage compositor instances
compositor: move the main() to a new weston.c file
build a libweston.so used by the weston binary
libweston: allow compositors to define the logging behavior
libweston: move the child process launching and monitoring to weston

libweston still internally uses those functions, so it is not linkable
yet by other compositors.
libweston: handle the screenshoot and record bindings in weston.c

Move the code launching the screenshooter client and implementing
the screenshot protocol out of libweston, and make screenshooter.c
a generic way to hook screenshoot protocols into weston.
text: build the text-backend in weston

It uses the weston_client_launch API, which is not available in
libweston.
libweston: expose the X11 backend's API
libweston: expose the drm backend's API
libweston: make a shared library for compositor launchers