Fix account and folder selection

[wmontwe]

The exsiting drawer has a complex setup to communicate account and folder changes between itself, the MessageList, MessageFragment and different entry points (widgets, notifications). To simplify the communication flow we decided to introduce a separate view model to keep track of the currently active account and when needed selected folder. It should be bound to the lifecycle of the MessageList activity and exposed to the navigation drawer as single source of truth.

[wmontwe]

Update: The view model approach caused a couple of lifecycle issues and a mutable state is used solely for the new drawer implementation. To avoid rewriting the MessageList, it will remain responsible to set the desired states.