More statusbar cleanup

[ian-r-rose]

Continuing #5352. Overall, this is mostly restructuring and cleaning up, with very little functionality change. It is, however, a lot of restructuring in an attempt to be more idiomatic.

• Adds docstrings.
• Removes unused code. Despite a large number of docstrings, this is a net-negative line diff.
• More restructuring and shuffling of things into a more modular/decoupled setup. Status item components are in the most relevant package. Plugins providing those components are in ${package}-extension.
• Several bugfixes.
• One breaking change: the tabs-vs-spaces status item no longer applies to notebooks: I found the user-interface to be pretty confusing and a bit buggy. It was a major simplification to only target file editors, the same as the settings menu does. These settings can still be set via the advanced settings editor.

[ian-r-rose]

Okay, this is ready for a look. I still want to do a few things, but probably in a follow-up.

TODO:

1. Add a bunch of statusbar tests.
2. There are a couple of dependencies that I am still not happy about. Think about how to break them.
3. Consider having the statusbar themed.

[blink1073]

Wow, that is quite a bit of plumbing, nicely done!