Move the functions launching clients to main.c

Move the functions launching clients to main.c

They belong in the compositor rather than libweston since they
set signals handlers, and a library should not do that behind its
user's back. Besides, they were using functions in main.c already
so they were not usable by other compositors.

Signed-off-by: Giulio Camuffo <giuliocamuffo@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>