WIP: lib: use a struct for cbs (Ask for preferences)

I would like to change all LibVLC functions using functions pointers to using a struct of function pointers.

• libvlc_audio_set_format_callbacks
• libvlc_audio_set_callbacks
• libvlc_video_set_output_callbacks
• libvlc_media_new_callbacks

The final goal is to remove the event manager and have libvlc media/mediaplayer/medialist/parser using the same struct mechanism, passed to their constructors.

OK with that ?

One other question: Contrary to VLC Core, I decided to copy the struct and not reference the static const struct pointer *. I think it's more high level this way (and more error-prone). But I'm also open to the Core way, as you want !