Variables view should show that it is waiting for the running cell

[digitarald]

Expected behaviour

When using the Variables view, clarify that when it needs to wait for the current cell to finish.

Actual behaviour

For many minutes, I am starting at this unhelpful loading message. I assume the kernel can't be pinged until the cell is done running.

Steps to reproduce:

While a long running cell runs, open the Variables view the first time.

[amunger]

closing in favor of microsoft/vscode#165445