libsaria: Added TRACK_LOADED callback

This is triggered whenever a new track is loaded.

include/libsaria/callback.h

@@ -5,6 +5,7 @@ enum callback_t {
 	PLAY,
 	PAUSE,
 	VOLUME,
+	TRACK_LOADED,
 	IDLE_TASK_QUEUED,
 	LIBRARY_REFRESH,
 };

libsaria/audio/controls.cpp

@@ -55,6 +55,7 @@ namespace libsaria
 		reset();
 		cur_file = file;
 		g_object_set(G_OBJECT(get_player()), "uri", uri.c_str(), NULL);
+		trigger_callback(TRACK_LOADED);
 		play();
 	}