Persist last opened collection

[kwerber]

Partially depends on #206

Is your feature request related to a problem? Please describe.

Trufos eventually needs to persist state that exists outside of collections. For example, we need to persist the information which collection the user has opened last. We probably also need to persist other information in the future. There is currently no functionality in our backend to do this.

Describe the solution you'd like

The backend needs a way to persist this kind of information. In my opinion it makes sense to separate this from the existing persistence service. Maybe the persistence service even needs to be renamed as it would no longer be the only service that is concerned with persisting information to the disk. The implemented solution should leave us space to add other information in the future, should the need arise.