Chat window isn't updated with MAM messages
Bug description
If MAM messages are received for a contact while a chat window for that contact is already open, those messages aren't shown until the window is reopened.
Steps to reproduce
Use a server that supports MAM, let Gajim go offline (or quit) while keeping a chat window open, send a message to that peer from another resource, and then come back online with Gajim. Gajim will receive the message via MAM, but it won't show the message before the chat window is reopened.
(Tested with the gajim_0.16
branch, changeset 15634:11ae397e6cc1
.)