Subscribing to Node state changes

[ctrueden]

Right now, the properties panel in sciview can get out of sync with node state from the scene. It would be awesome to have a callback mechanism (NodeListener) to be told whenever details of a node change. Then, in sciview, we can subscribe to node changes and update the properties panel accordingly. This would be useful in other scenarios as well, really any time you want to keep track of what's happening with nodes. See also this Zulip topic.

[ctrueden]

@skalarproduktraum This issue came up again during the sciview hackathon with @kephale back in July, and I expect will continue to crop up in future, in scenarios where people want to keep their own data structures synchronized with scenery state. What do you think—is a NodeListener or similar something that would be pretty straightforward to weave into the codebase? Or are there obstacles?