compositor: add API to manage compositor instances

This commit adds three new exported functions:
- weston_compositor_create() returns a new weston_compositor instance,
initializing it as the now removed weston_compositor_init() did.
- weston_compositor_exit(compositor) asks the compositor to tear
down by calling the compositor's exit vfunc which is set by the
libweston application.
- weston_compositor_destroy(compositor) is called by the libweston
application when tearing down the compositor. The compositor is destroyed
and the memory freed.

Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>