Refactor entity management

[zehnm]

The separate backing lists for available and configured entities makes it more difficult for the integrations to properly manage entity states and attributes.
Furthermore, the get_available_entities should return all entities, no matter if the Remote Two has subscribed to certain entities or not. (This probably needs some clarifications in the WebSocket Integration-API as well).
There should only be one backing list with either additional attributes or metadata holding the subscription state.