Widgets: Insert widgets into the selected sidebar correctly in accessibility mode

[t-hamano]

Trac ticket: https://core.trac.wordpress.org/ticket/64380

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props wildworks, sabernhardt, joedolson.

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[github-actions]

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[joedolson]

This looks good to me!

[t-hamano]

Thanks both of you for the reviews! I'll commit this PR.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61399
GitHub commit: 9074e4e

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.