quick frontend(QuickApp): added getDefaultAvatar method:

this method (not implemented in quick frontend) should facilitate handling of entities without avatar

frontends/src/quick_frontend/quick_app.py

@@ -829,6 +829,14 @@ class QuickApp(object):
 
         self.bridge.launchAction(callback_id, data, profile, callback=action_cb, errback=self.dialogFailure)
 
+    def getDefaultAvatar(self, entity=None):
+        """return default avatar to use with given entity
+
+        must be implemented by frontend
+        @param entity(jid.JID): entity for which a default avatar is needed
+        """
+        raise NotImplementedError
+
     def disconnect(self, profile):
         log.info("disconnecting")
         self.callListeners('disconnect', profile=profile)