Update to audioplayers v4

Author: DoumanAsh

include/plugins/audioplayers.h

@@ -30,6 +30,8 @@ void audio_player_set_volume(struct audio_player *self, double volume);
 
 void audio_player_set_playback_rate(struct audio_player *self, double rate);
 
+void audio_player_set_balance(struct audio_player *self, double balance);
+
 void audio_player_set_position(struct audio_player *self, int64_t position);
 
 void audio_player_set_source_url(struct audio_player *self, char *url);