Remove sorting for displayed multi-value tags and provide a sort tags plugin

Picard stores tags unsorted but displays them sorted and this is
inconsistent.

This commit removes display sorting of multi-value tags from Picard
improves consistency by displaying what will be / is actually saved.

This commit also provides a plug-in to allow user to sort tags both
displayed and stored in the file.

contrib/plugins/sort_multivalue_tags.py

@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+
+# This is the Sort Multivalue Tags plugin for MusicBrainz Picard.
+# Copyright (C) 2013 Sophist
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+PLUGIN_NAME = u"Sort Multi-Value Tags"
+PLUGIN_AUTHOR = u"Sophist"
+PLUGIN_DESCRIPTION = u'Sort Multi-Value Tags e.g. Release Type, Lyrics alphabetically.'
+PLUGIN_VERSION = "0.1"
+PLUGIN_API_VERSIONS = ["0.15"]
+
+from picard.metadata import register_track_metadata_processor
+
+# Define and register the Track Metadata function
+def sort_multivalue_tags(tagger, metadata, track, release):
+
+	for tag in filter(lambda x: len(metadata[x]) > 1, metadata.keys()):
+         data = metadata.getall(tag)
+         if len(data) > 1:
+             sorted_data = sorted(data)
+             if data != sorted_data:
+                metadata.set(tag,sorted_data)
+
+register_track_metadata_processor(sort_multivalue_tags)

picard/ui/metadatabox.py

@@ -99,11 +99,9 @@ class TagDiff:
 
     def add(self, tag, orig_values, new_values, removable):
         if orig_values:
-            orig_values = sorted(orig_values)
             self.orig.add(tag, orig_values)
 
         if new_values:
-            new_values = sorted(new_values)
             self.new.add(tag, new_values)
 
         if orig_values and not new_values: