libsaria: Add sources.get_cur_id()

I think a function for finding the current song id will be useful if I
want to remove the old get_attrs() function...

libsaria/sources/__init__.py

@@ -78,6 +78,9 @@ def all_attrs():
 		res["count"] = attrs.get_count("count")
 	return res
 
+def get_cur_id():
+	return cur_id
+
 def filter(text):
 	visible = library.filter(text)
 	playlist.set_visible(visible)