Save a user's chosen, persistent nickname in the MUC roster data as 'alt_nick'

The use of <nick /> elements in MUCs is now discouraged in XEP-0172, however.

sleekxmpp/plugins/xep_0045.py

@@ -156,6 +156,7 @@ class XEP_0045(BasePlugin):
         entry = pr['muc'].getStanzaValues()
         entry['show'] = pr['show']
         entry['status'] = pr['status']
+        entry['alt_nick'] = pr['nick']
         if pr['type'] == 'unavailable':
             if entry['nick'] in self.rooms[entry['room']]:
                 del self.rooms[entry['room']][entry['nick']]