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.
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