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
libweston: expose the wayland backend API
libweston: libweston-ify xwayland

This creates a new library, libweston-xwayland.so with the entry point
to easily spawn a xwayland server. The xwayland module now links to that
library.