rename confusing libvlc stat API(s)

We now have

libvlc_media_get_stat
libvlc_media_get_stats

These APIs have a single character difference, yet are about widely different things.

We should rename one or the other function as the current state of things is widely confusing IMHO. Suggestions welcome.

• libvlc_media_get_stat to libvlc_media_get_filestat

or

• libvlc_media_get_stats to libvlc_media_get_playback_stats

or

• libvlc_media_get_stats to libvlc_media_get_player_stats

or move the statistics to the media_player entirely.