Allowed for linking FocusTracker instances together and propagate the focus state

[oleq]

Suggested merge commit message (convention)

Fix (ui): The dropdown menu component should not cause an editor blur if used in a BalloonToolbar while the user hovers a nested menu. Closes #17277.

Feature (utils): Made FocusTracker extendable with other FocusTracker instances to allow logical focus tracking across separate DOM sub-trees.

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Additional information

Closes https://github.com/cksource/ckeditor5-commercial/issues/6633.

Comes with https://github.com/cksource/ckeditor5-commercial/pull/6672

TODOs:

• Remove FT#_label debug lines from features (after QA)
• Update docs.
• Migrate comments from The dropdown menu component should not cause an editor blur if used in a BalloonToolbar #17140.

[scofalik]

I have to admit that full and deep code review would take a lot of my time, trying to get to the roots of how exactly this work.

I added some comments to places which weren't immediately obvious to me, but after going through it for the second time, I think I understood why they are like that. It seems to me that introduced changes make sense.

But I cannot tell whether something is missing, or some other place should be modified too.

I know you put a lot of (hehe) focus into this issue and this is a third (?) try to get it working in a way that won't introduce breaking changes. So I think I have to lean on your experience in this matter.

Happy to talk about the bits which I commented on, I wonder if I got it right.

[scofalik]

Please finish the todos and the one thing regarding the private member, and it should be fine.