Textbuffer: add "find last message" function

poezio/tabs/muctab.py

@@ -161,7 +161,7 @@ class MucTab(ChatTab):
             delta = datetime.now() - self.last_connection
             seconds = delta.seconds + delta.days * 24 * 3600
         else:
-            seconds = None
+            seconds = self._text_buffer.find_last_message()
         muc.join_groupchat(
             self.core,
             self.jid.bare,

poezio/text_buffer.py

@@ -277,6 +277,13 @@ class TextBuffer:
     def del_window(self, win) -> None:
         self._windows.remove(win)
 
+    def find_last_message(self) -> Optional[Message]:
+        """Find the last real message received in this buffer"""
+        for message in reversed(self.messages):
+            if isinstance(message, Message):
+                return message
+        return None
+
     def __del__(self):
         size = len(self.messages)
         log.debug('** Deleting %s messages from textbuffer', size)