Add statusbar

[saulshanabrook]

Closes #5352 by adding statusbar to core

To create initial commit (from this guide on https://medium.com/@porteneuve/mastering-git-subtrees-943d29a798ec):

git checkout master
git remote add statusbar https://github.com/jupyterlab/jupyterlab-statusbar.git
git checkout -b statusbar
git subtree add --prefix=packages/statusbar-extension statusbar master
git push --set-upstream --no-verify origin statusbar

Then we added the name to some shared spots and edited some config files in it to make them more homogenous to the rest of the extensions.

We can merge this if the tests all pass and then work on splitting sone of the core logic into a @jupyterlab/statusbar package and moving out the registration of statusbar items to their respective extensions.

[ian-r-rose]

This is a huge PR, and I agree we should merge ASAP and do cleanup/splitting in follow-ups. Thanks @saulshanabrook!

[saulshanabrook]

Thanks @ian-r-rose for the cleanup

[blink1073]

Thanks!

[dhirschfeld]

Just confirming that for 0.35.2 we will no longer need to install the statusbar extension separately?

[ian-r-rose]

I don't think this will be in a patch release, there is some more work to be done to further integrate this into the core.

[dhirschfeld]

Thanks for the info! I'll keep an eye on the releases then.

Also something to consider if I build from master - I've been considering spinning up a dev environment.