libsaria: Rename playlist's next() function

This is to prepare for creating a new file "next.py" dedicated to
choosing the next song.  I also use this as an opportunity to pass the
current song id to the next function.

libsaria/sources/__init__.py

@@ -65,7 +65,7 @@ def next():
 	global cur_id
 	cur_id = queue.next()
 	if cur_id == None:
-		cur_id = playlist.next()
+		cur_id = playlist.get_next(cur_id)
 	return library.get_attrs(cur_id, "filepath")
 
 def list_source(source, *attrs):

libsaria/sources/playlist/__init__.py

@@ -106,7 +106,7 @@ def play_id(id):
 		return True
 	return False
 
-def next():
+def get_next(cur_id):
 	id = None
 	if len(playlist.song_list) == 0:
 		return