Walk filtered IDs

We already have a function for walking all song ids.  Collections should
be able to walk the filtered IDs too.

libsaria/collection/collection.py

@@ -92,6 +92,10 @@ class Collection:
 		for tag in self.tag_tree.walk_forwards():
 			yield tag[3]
 
+	def walk_filtered_ids(self):
+		for id in self.visible:
+			yield id
+
 	def find_id(self, file):
 		stripped = file.strip(libsaria.path.sep)
 		split    = stripped.split(libsaria.path.sep)