Improve invalid navigation menu links

[talldan]

What problem does this address?

When testing, I couldn't get the feature introduced in #31716 to work correctly.

These tooltips never seems to display:

• https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/navigation-link/edit.js#L752
• https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/navigation-link/edit.js#L817

They're supposed to show the text "This item has been deleted, or is a draft" or "This item is missing a link", which seems important. The problem is that the tooltips are anchored to React Fragments so won't ever be positioned, and that seems to have been the case since the introduction of the feature (you can see the tooltips never appear in the gifs in the PR).

It also doesn't consider a link to a post that has been deleted as invalid, only one that's currently in the trash.

Another problem is that if a link to a draft is present, the label isn't editable. This is something a user would want to do if they're in the process of launching a new page on their website. It seems like this was a late addition to the PR (#31716 (comment)), so probably wasn't tested fully.

Also note that there's a separate issue around this feature in that it can spawn a lot of individual HTTP requests (#42904).

What is your proposed solution?

I did see whether fixing the tooltip would be a quick thing, but IMO it doesn't look good given the way it overlaps the toolbar. It looks very cluttered:

It might be better to move the more detailed status information into the LinkControl popover.