Increase efficiency of Python values when they're expanded in the Variables pane

[jmcphers]

Currently, Python variable values do not auto-update in the Variables pane the way R variables do. This is because it is difficult and inefficient to recursively crawl all of the variables to check for updates.

One way to make this better might be to maintain a list of variables that are currently open (i.e. an array of expanded paths) on the backend as well as the frontend. This has the disadvantage of making the backend more stateful, but it also has a significant advantage: instead of trying to inspect every variable to see if it has changed, we could just crawl the ones that are currently open for inspection, which could probably be done efficiently.

[seeM]

This is working on the latest release, although it isn't efficient so most of the notes in the issue description are still valid. Maybe we can reword the title?

[isabelizimm]

afaict, we haven't had anyone complain about Variables pane speed and this is a significant change to how to Variables pane works. Do we want to push this to post-RC?