Show joined rooms, delete left rooms

To make the models update correctly in QML:
- ListModel and _QtModel merged
- Return a ListModelMap QObject from properties instead of
  a DefaultDict → QVariantMap

harmonyqml/backend/model/__init__.py

@@ -1,6 +1,7 @@
 # Copyright 2019 miruka
 # This file is part of harmonyqml, licensed under GPLv3.
 
-from .list_model import ListModel, _QtListModel
+from .list_model import ListModel
+from .list_model_map import ListModelMap
 from .qml_models import QMLModels
 from . import enums, items