Window session mechanism

[daemontus]

When talking between UI and backend, we need to manage window-specific state. For this we need some concept of "session".

[ondrej33]

The core mechanisms were introduced in #23. These covered most of the currently used functionality and already allowed for multiple sessions, but at that time, the app only consisted of a single editor session. Some modifications were done in #47 to allow for a new analysis session, and for inter-session communication. However, we are still far from the final product.

Some refactoring of the session-management mechanism should happen, perhaps once we have a more mature analysis workflow. We should also look into how to properly handle the closing of sessions, as I think this is currently not tackled.