Use same autosave heuristic when closing widget

[colin-grant-work]

What it does

Fixes #15501 by using the same heuristic for autosaveability when closing a widget as when deciding whether to apply autosaving.

How to test

1. Set your autosave preference to something other than off
2. Create an untitled editor and populate it.
3. Attempt to close it.
4. You should be offered the 'Change will be lost' dialog (not straight to the save location dialog).
5. Choose any of the options, and you should get the expected behavior.

Note

If you choose to save, the newly saved version will be opened, rather than both being closed. Perhaps that should be otherwise?

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

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

[sdirix]

It works for me, however I have a comment.

[sdirix]

Thank you!