When a variable in the variable explorer is updated, update open dataviewers

[jmew]

If you have a dataframe open in the data viewer, and modify the dataframe in the notebook, the changes dont get propagated to the data viewer unless you close and open it again

[DonJayamanne]

We might want to be careful with auto updating variables and the like.
Plenty of people have had issues with a similar variable viewer in notebooks (and still having issues). The root cause was identified to be with the variable viewer, causing huge delays/perf issues in jupyter notebooks.

Also, we know its a little slow to load with large data sets, hence auto loading it might end up being counter productive.

[rchiodo]

The not getting propagated was supposed to be on purpose. That way you could see a before and after picture of the variable. We should put the execution count or something in the title though.

[greazer]

Couple of thoughts. Rename to be "Snapshot viewer", or add a refresh button, or add some other form of messaging.

[adrimos]

A refresh button would be great!

[greazer]

Dupe of #1143