notebook output optimization

[jonah-iden]

What it does

Changes the notebook output rendering to only use a single webview for each editor.
This should also fix some previously existent issues like with moving cells having outputs and changing mime type.

How to test

Open a notebook and test that everything regarding rendering of outputs still works.

• opening notebooks with outputs
• executing cells
• moving/deleting cells with outputs
• creating new cells infornt/in between cells with outputs
• collapsing outputs
• clearing outputs
• chainging output presentation type

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Really impressive! The notebook editor is now way more responsive and a lot of the bugs (like output disappearing on moving cells, etc.) are now fixed. However, I found some issues/regressions with the changes:

1. Some outputs cannot be rendered after reopening a notebook. I don't think we can fix this (VS Code also cannot, see this image). However, this leads to miscalculations in the output renderer logic and everything seems to be misaligned:
   
2. Clicking on a rendered Markdown cell (non-editing mode) does not focus the cell. One has to click on the sidebar of the cell instead. AFAIK this is a regression from the current master branch.
3. Closing a notebook editor takes a surprising amount of time. In some cases more than a second. During this time, the application seems to freeze up. I can pretty consistently reproduce this using this notebook.

[msujew]

Continuing the issues above:

4. I've noticed some jittering in the layout while scrolling through a larger notebook. The editors seem to have a "second layout" phase where they increase their size by a few pixels. This leads to a lot of jittering during the scrolling due to resizing editors. IIRC this is also the case on master, so treat this as a low-prio issue for now.

[msujew]

I believe I found another (pretty weird) issue:

1. Open a notebook and clear one cell output (the document becomes dirty)
2. Close the notebook (it doesn't matter whether you save the changes)
3. Reopen the notebook - no outputs are visible
4. Closing and reopening the notebook fixes the issue (?)