Create demo for an undo/redo stack

[daemontus]

We are planning an event-driven GUI where the frontend sends events to the backend where all state is maintained. These events pass through an undo/redo stack where they are stored. Ideally, the payload of these events is typed such that we can verify the integrity of the payloads from TypeScript as well (using serde-generate or something similar)

Here, we need to:

• Define some kind of global "app state" object. This object should be "addressable" using labels/paths to ensure the events can reference the objects which they modify.
• Define an "event payload" type, and an event struct that can be used to apply this payload to the app state.
• Define the undo/redo stack.

[ondrej33]

This is implemented thanks to #23.