Progress API to report status of computation to the GUI

[JaroslavTulach]

I see progress field in Pending update. We could display node as being evaluated if progress != 0. If you know better API for this, please let me know.

Yes, that was my idea, when I introduced the progress field. Thus it'd be:

• if there is no progress field, the node is pending and not yet being evaluated
• if there is some value in progress field the node is pending and being evaluated

All that's needed is the IDE to use such info and language server to fill it.

[JaroslavTulach]

• depends on State and Contexts are not preserved through the Polyglot boundary #7117
• as progress collector should be associated with a State
• we need the State to "survive" callback to Java and back to Enso