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/AccountDelegate.qml

@@ -21,7 +21,7 @@ ColumnLayout {
 
             Base.HLabel {
                 id: "accountLabel"
-                text: display_name
+                text: display_name.value || user_id
                 elide: Text.ElideRight
                 maximumLineCount: 1
                 Layout.fillWidth: true