Video: Fix track editor state saves without explicitly applying the changes

[yogeshbhutkar]

What?

Follow-up to #70227 (comment)

This PR implements an internal editing state for SingleTrackEditor and uses the onChange to write the state to attributes only when Apply is pressed.

Why?

Previously, for every change in value, the onChange would execute, writing the state changes to attributes as they occurred; this rendered the Apply button useless.

Removing the Apply button itself was previously discussed in the tagged PR, but it was decided to keep it for better Accessibility and UX.

How?

An internal state is maintained, and the state is synced with attributes when the changes are applied; otherwise, they are discarded.

Testing Instructions

1. Create a Video block.
2. Attach a Track to it by selecting a subtitle file from the Media Library.
3. Fill in the details. Do not click Apply and then close the modal.
4. Reopen the single-track editing modal, and confirm that the states are reverted.

Testing Instructions for Keyboard

Same.

Screencast

pr-demo.mov

[yogeshbhutkar]

I intended to omit the language-specific srcLang attribute from the previous implementation; however, according to the MDN reference:

If the kind attribute is set to "subtitles", then srclang must be specified.

Even when the kind attribute is omitted, it defaults to "subtitles", which still requires srclang to be defined.

[Mamaduka]

Makes sense 👍

[yogeshbhutkar]

@Mamaduka, can we move this PR forward if the implementation looks good? 😅

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: yogeshbhutkar <yogeshbhutkar@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[Mamaduka]

Thank you, @yogeshbhutkar!

The refactoring works as expected ✅