lib: change libvlc_media_discoverer_t creation

libvlc_media_discoverer_new_from_name was creating a services_discovery_t and
was starting it, so libvlc_MediaDiscovererStarted event (or any other events)
could not be received.

To fix that, Split libvlc_media_discoverer_new_from_name into
libvlc_media_discoverer_new and libvlc_media_discoverer_start. That way, we can
attach events between create and start.

libvlc_media_discoverer_new_from_name is now deprecated, but it still works
like before.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>