[WIP] Refactor statusbar

[ian-r-rose]

Finishes #5352. This is a big restructuring of the status bar extension to be more idiomatic and in line with the rest of the core codebase.

1. Breaks the statusbar extension into two packages, statusbar and statusbar-extension.
2. Removes the IDefaultsManager. This was mostly redundant with the actual IStatusBar interface, and I found it pretty confusing. Extension authors should add status items using the IStatusBar. One downside of this is that the IDefaultsManager handled some logic about enabling and disabling status items via the settings registry. We may want to revisit that, but at the moment all of the status items are individual plugins, so they can be disabled at the application level.
3. Moves several of the status bar items to other packages, where appropriate. For instance, the notebook trust status item is moved to the notebook-extension. Several of the status items that are more cross-cutting in their application remain in the statusbar-extension.
4. Removes and simplifies various things.
5. Adds a lot of docstrings.

[blink1073]

Still WIP?

[ian-r-rose]

Yeah, I think so. It's an improvement right now, but I still want to hack at it a bit.

[ian-r-rose]

Though I suppose we could merge and iterate, since this would be a painful rebase...

[blink1073]

👍 for iteration

[ian-r-rose]

Okay, merge away then!

[blink1073]

Done, thanks!