🪴external rendermime management

[stevejpurves]

in #630 I was discussing how to improve the current very buggy rendermime creation. I pushed though on option (3) [making rendermime registry creation an explicit part of the API] in this PR, and...

Things are immediately much better, specific improvements are:

• much easier to reason about the expected ipywidgets behaviour
• some confusing addWidgetRenderer calls in notebooks have been removed
• the ThebeManager implementation is now just simple without the bizzare behaviour of adding widgets to some other rendermime.
• the only place where a rendermime registry is completed is now in the PassiveCellRenderer which feels right as that class is intended for standalone usage on static content.

So this is a keeper 🤘

[github-actions]

PR Preview Action v1.4.4
Preview removed because the pull request was closed.
2023-05-31 15:42 UTC

[rowanc1]

This is awesome to see, really like the simplifications and explicit passing of the rendermime!