Fix state restoration in OutputWidget

[colin-grant-work]

What it does

Fixes #11923

How to test

1. Open the output widget and select a certain output channel.
2. Reload the application
3. That channel should be selected if available, and its content should be shown, if there is any.
4. If it is not available (because of conditional extension activation, e.g.: try opening a TypeScript file, selecting the TS extension channel, then closing the file)...
   1. If you select another channel before the restored channel becomes available, that channel should be retained.
   2. If you do not explicitly select another channel before the restored channel becomes available, that channel should be selected when available.
5. Close the output widget and reopen it in the same session.
6. The same channel should be selected.

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
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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

[ndoschek]

Thanks Colin, changes LGTM and the state restoration works great for me!