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/client_manager.py

@@ -65,6 +65,7 @@ class ClientManager(QObject):
     def _on_connected(self, client: Client) -> None:
         self.clients[client.userID] = client
         self.clientAdded.emit(client)
+        client.startSyncing()
 
 
     @pyqtSlot(str)