Add ability to language-switch between TS and Python

[Zlatkovsky]

(When settings are set for jupyter)

[Zlatkovsky]

@shaofengzhu , there is now a language-switcher in the bottom right (which before simply displayed the language but didn't let you switch it)

For it to become a dropdown, note that you'll need the appropriate settings already set. I.e.:

jupyter.url: 'http://localhost:8888'
jupyter.token: exceljupyter
jupyter.notebook: HelloWorld.ipynb

[Zlatkovsky]

@shaofengzhu , I will merge this in tomorrow morning. In the meantime, you can either cherry-pick my commit into yours (but it might get messy), or just tweak the YAML like we did before (to have it say language: python, just like in https://gist.github.com/Zlatkovsky/3890dae8bebb53658e04efbbb6f3f46d.

I'll get it into alpha before tomorrow's demo, though.

@nico-bellante , since this had to do with actions and selectors and etc, would appreciate your review -- though it's also fine if you want to give your feedback after I check in (since I want this in alpha for the demo tomorrow anyway), and I can fix it later.

[Zlatkovsky]

@nico-bellante , going ahead and merging. But would still appreciate your post-factum review if you have the time.