Proper display name retrieval implementation

For any name not found in rooms data, rely on new
nio.HttpClient.get_displayname() function to get and cache it,
e.g. for our own name if no room is joined and past events from users
who left the room.

@futurize now returns PyQtFuture objects, wrapper for the
concurrent.futures.Future objects that can be used from QML,
to ensure name retrieval does not block the GUI.

harmonyqml/backend/model/items.py

@@ -5,10 +5,12 @@ from typing import Dict, List, NamedTuple, Optional
 
 from PyQt5.QtCore import QDateTime
 
+from ..pyqt_future import PyQtFuture
+
 
 class User(NamedTuple):
     user_id:        str
-    display_name:   str
+    display_name:   PyQtFuture
     avatar_url:     Optional[str] = None
     status_message: Optional[str] = None