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/components/side_pane/utils.js

@@ -15,7 +15,7 @@ function get_last_room_event_text(room_id) {
 
     if (! found) { return "" }
 
-    var name          = Backend.getUser(ev.dict.sender).display_name
+    var name = Backend.getUserDisplayName(ev.dict.sender, false).result()
     var undecryptable = ev.type === "OlmEvent" || ev.type === "MegolmEvent"
 
     if (undecryptable || ev.type.startsWith("RoomMessage")) {