Get artist and album lists from the library

This patch makes it easy to get an artist or album list out of the
library.

libsaria/sources/library.py

@@ -71,6 +71,23 @@ def walk():
 def file_to_id(file):
 	return os.stat(file).st_ino
 
+def artists():
+	keys = tag_tree.keys()
+	keys.sort()
+	res = []
+	for key in keys:
+		res.append( (tag_tree[key].value, key) )
+	return res
+
+def albums(artist):
+	art = tag_tree[artist]
+	keys = art.keys()
+	keys.sort()
+	res = []
+	for key in keys:
+		res.append( (art[key].value, key) )
+	return res
+
 def get_attrs(id, *attrs):
 	res = []
 	rec = tracks.get(id, None)