LibVLC 4.0 API

Here are all the steps to finish the LibVLC 4.0 API

• move the thumbnailer from libvlc_media_t to libvlc_thumbnailer_t
• move the parser from libvlc_media_t to libvlc_parser_t
• add libvlc_downloader_t
• Remove events from libvlc_media_t and libvlc_media_list_t. New/Delete subitems events should be propagated from the libvlc_discoverer_t and libvlc_parser. That way, these 2 classes won't recquire a libvlc_instance_t or a libvlc_event_manager_t
• Replace all existing callbacks functions with a (const struct foo_cbs *, void *opaque) mechanism
• Replace all existing event with a (const struct foo_cbs *, void *opaque) mechanism

cf. #27841 (closed)