Revert "Revert "tabs/privatetab: Fixes #3491: Add MUC <x/> marker in MUC-PMs. Thanks Ge0rG.""

This reverts commit e9a6ea7a59.

poezio/tabs/privatetab.py

@@ -13,6 +13,7 @@ the ConversationTab (such as tab-completion on nicks from the room).
 import curses
 import logging
 from typing import Dict, Callable
+from xml.etree import cElementTree as ET
 
 from slixmpp import JID
 
@@ -152,6 +153,8 @@ class PrivateTab(OneToOneTab):
         )
         msg['type'] = 'chat'
         msg['body'] = line
+        x = ET.Element('{http://jabber.org/protocol/muc#user}x')
+        msg.append(x)
         # trigger the event BEFORE looking for colors.
         # This lets a plugin insert \x19xxx} colors, that will
         # be converted in xhtml.