Move command helpers to a better place

[Reinmar]

See ckeditor/ckeditor5-core#10 (comment).

E.g. I noticed that none of these functions have anything to do with the core. They only know about the engine, so it seems that they can be moved there. Question – where precisely?

[Reinmar]

We talked with @pjasiun that those helpers would make perfect sense in the Schema.

[Reinmar]

I've been also thinking what to do with ToggleAttributeCommand. It's the only command which we added to the core but only the basic-styles package is using it.

In CKE4 the style system was used heavily by a wide range of features, but that won't be the case in CKE5. Instead of building a centralised super system for dealing with block/object/inline styles, we have an abstract model and simpler utils spread around the engine (mostly).

Hence, ToggleAttributeCommand makes no sense here. I'd move it to basic-styles. If one will want to create similar command to any of our basic-styles, they should depend on basic-styles, which is fine because those commands will be thematically related anyway.

[Reinmar]

BTW, besides basic styles there's also LinkCommand. In CKE4 they were all based on the style system but, of course, there was a problem with the style system's flexibility (cause link is a bit different than typical styles) so we had this lovely line:

https://github.com/ckeditor/ckeditor-dev/blob/621199c174b8df22306ca8c55742a676d8db3fe0/plugins/link/dialogs/link.js#L36

style.type = CKEDITOR.STYLE_INLINE; // need to override... dunno why.

I love the fact that this line is 9 years old :D

[Reinmar]

Related ticket in the engine: https://github.com/ckeditor/ckeditor5-engine/issues/969.

[Reinmar]

Another ticket – now basic-styles: https://github.com/ckeditor/ckeditor5-basic-styles/issues/47.

[Reinmar]

And one more in the link package: https://github.com/ckeditor/ckeditor5-link/issues/131.

[Reinmar]

And I can also see that getSchemaValidRanges() is used in ckeditor5-autoformat, but it shouldn't be used there. I'll try to fix https://github.com/ckeditor/ckeditor5-autoformat/issues/7 and the dependency will be gone.

[Reinmar]

I had to give up with https://github.com/ckeditor/ckeditor5-autoformat/issues/7 for now, so I'll just remove the dependency on the core in https://github.com/ckeditor/ckeditor5-autoformat/issues/31.

[Reinmar]

All done, so we can remove these helpers from this package.