Declarative nested toolbars

[oleq]

Suggested merge commit message (convention)

Feature (ui): Allowed grouping toolbar items into drop-downs by using a declarative config. Closes #12490.

BREAKING CHANGE (basic-styles): The bold icon has been moved to the ckeditor5-core package.
BREAKING CHANGE (paragraph): The paragraph icon has been moved to the ckeditor5-core package.

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Additional information

TODOs:

• Are we OK that the bold and paragraph icons moved to the ckeditor5-core package (e.g. while italic and heading1 didn't)?
• Docs proofreading.
• Do we want a default icon? (three vertical dots is a solid candidate)

[mlewand]

@oleq regarding moving the icons, why core package? Wouldn't UI package be more fitting?

Regarding the default icon, where do you see it kicking in? Would that it's added as an opt-out (so that integrator has to pass icon: false to remove the icon)? Or would it be present if integrator didn't provide neither the icon nor withText property?

[oleq]

@oleq regarding moving the icons, why core package? Wouldn't UI package be more fitting?

Because we're already collecting icons there. Alignment icons are there, object alignment icons too. And blockquote. All icons that could be (or already are) re-used by other features.

I don't say this is the best place, though. Honestly I don't know why we have icons scattered across the project. They don't make sense without ckeditor5-ui and the IconView class anyway. Maybe because sb. else could have their custom UI but use our icons? I don't know (remember), really. I'd rather keep all icons in the project in the ckeditor5-ui package, but this would be a huge change. It probably overlaps with #12496 too.

Regarding the default icon, where do you see it kicking in? Would that it's added as an opt-out (so that integrator has to pass icon: false to remove the icon)? Or would it be present if integrator didn't provide neither the icon nor withText property?

The latter. But this is not critical I guess. There are warnings logged if you fail to provide either an icon or withText: true, anyway.

[godai78]

I made some changes, as we use "drop-down" (hyphenated) when it's an adjective, and here we mostly had nouns.