quick_frontend, primitivus: various fixes (MUC and cached signals):

- actually call the cached signals, everything was there but the call was not done
    - display '[]' instead of '[None]' when group message coming from the room (no resource)
    - catch the exception when trying to delete an occupant who's not in the room
    - do not cache a "main resource" for MUC entities

frontends/src/quick_frontend/quick_app.py

@@ -388,6 +388,7 @@ class QuickApp(object):
         cached_signals = self.signals_cache.pop(profile, [])
         for function_name, handler, args, kwargs in cached_signals:
             log.debug(u"Calling cached signal [%s] with args %s and kwargs %s" % (function_name, args, kwargs))
+            handler(*args, **kwargs)
 
         self.callListeners('profilePlugged', profile=profile)