Regions: move publish button etc. to publish region

[ellatrix]

Description

This PR moves the publish, save, preview, and setting buttons to the publish region in the DOM. Visually, they remain in the same place.

Why is this better?

• Visually the page is split in two columns: the big editing area and the sidebar. The publish button is displayed in the second column, so the tabbing order makes sense.
• It's also a good logical order: write content, then publish. See Accessibility feedback: the Publish button placement is confusing #3697. Currently there's even a shortcut to the publish panel after the content. With this change, the shortcut is no necessary.
• When the "Top toolbar" is enabled, reverse tabbing should bring you to the block toolbar, not the publish toolbar.
• It's easier to access button such as undo and redo from the selected block.
• It's easier to access the save button from the selected block (tab once).
• It gets rid of a hack to hide the save button. This hack is now no longer needed since the buttons are in their right place.
• It will enable us to unify the block toolbar (top, mobile, contextual) and simplify things.
• The tab order is now very similar to the classic editor, where the publish button also appears after the content.

Now tab order also makes sense when the block toolbar (when the top toolbar is enabled) wraps downwards for smaller screens.

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .

[ellatrix]

After chatting with @MarcoZehe, we agreed that #19623 is a better direction to take.