libsaria: Add a VOLUME callback

This will be triggered whenever the volume changes through the libsaria
class.

include/libsaria/callback.h

@@ -4,6 +4,7 @@
 enum callback_t {
 	PLAY,
 	PAUSE,
+	VOLUME,
 };
 
 #endif /* LIBSARIA_CALLBACKS_H */

libsaria/libsaria.cpp

@@ -19,4 +19,5 @@ void Libsaria::load(string filepath)
 void Libsaria::set_volume(double volume)
 {
 	audio.set_volume(volume);
+	trigger_callback(VOLUME);
 }