Make the Dialog API depend on MediaPlayer instead of LibVLC instance

Enable the dialog API in multi-mediaplayer scenarios.

All LibVLC 4+ users.

Current state of the API

LIBVLC_API void libvlc_dialog_set_callbacks(libvlc_instance_t * p_instance, const libvlc_dialog_cbs * p_cbs, void * p_data)

Evolution of the API that would solve this use case

LIBVLC_API void libvlc_dialog_set_callbacks(libvlc_media_player_t * p_mp, const libvlc_dialog_cbs * p_cbs, void * p_data)

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