feat: Store trait for unified client data access and observability

[boxdot]

The new trait Store abstracts the access and modification of the
client data. Additionally, it adds observability to the changes in the
client data via the Store::subscribe method and the
StoreNotification type. The trait can be used as dependency inversion
between the app logic and the core client crates.

Currently, only the CoreUser implements the Store trait. The
observability is implemented via the StoreNotifier helper structure.
It is passed to the functions that modify data in the persistence layer.
The modifications are collected in a StoreNotification, and if the
corresponding operation succeeds, it is send to all current subscribers.

The cubits ConversationListCubitBase and
ConversationDetailsCubitBase now observe the changes in the client
data via the Store trait (instead of subscribing to the fetched
messages).

Closes #269
Closes #254

[raphaelrobert]

Thanks, this looks like a good starting point.