Engine should send notification about node status

[JaroslavTulach]

Pull Request Description

When nodes get invalidated in the cache, they have to be recomputed. Let the IDE know which of the nodes are pending and are going to be recomputed.

Important Notes

Right now the PR sends Api.ExpressionUpdate.Payload.Panic("pending..." as that is something the IDE can display. Proper Pending status will be subject of further changes.

Checklist

Please include the following checklist in your PR:

• [ ] The documentation has been updated if necessary.
• [ ] All code conforms to the Scala, Java, and Rust style guides.
• All code has been tested:
  • [ ] Unit tests have been written where possible.
  • [ ] If GUI codebase was changed: Enso GUI was tested when built using BOTH ./run ide build and ./run ide watch.