To-do lists in document lists

[niegowski]

Suggested merge commit message (convention)

Feature (list): Introducing the to-do lists compatible with the document list feature. Closes #14663.

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Additional information

For example – encountered issues, assumptions you had to make, other affected tickets, etc.

[charlttsie]

@niegowski It seems like with Heading 1, the position of the checkbox is too high compared to how it looks with unordered and ordered document lists:

It looks better with other heading levels:

Is should probably be aligned to bottom, as it is with other document list types:

[charlttsie]

Checkboxes next to code block items are not responsive

Steps

1. Add the Code block plugin to http://fake.ckeditor.com:8125/ckeditor5-list/tests/manual/todo-documentlist.html
2. Add a code block as a to-do list item
3. Try to check the checkbox

Result
The checkbox can't be checked:

checkbox-codeblock.mp4

Additional info
Works good with other widgets (including images, tables, block quotes)

[charlttsie]

Switching to-do list on and off on an element affects checkboxes of the list items below

Steps

1. Open http://fake.ckeditor.com:8125/ckeditor5-list/tests/manual/todo-documentlist.html
2. Add a couple of to-do list items and check the checkboxes for some of them
3. Place the caret in the first list item
4. Click the to-do list toolbar icon

Result
For a moment, checkboxes on unchecked list items get green highlight, while checked checkboxes lose the check mark:

checkboxes.mp4

Additional info
It's not reproducible with legacy to-do lists

[charlttsie]

Alignment of to-do document list items can't be changed

Steps

1. Add the alignment plugin to http://fake.ckeditor.com:8125/ckeditor5-list/tests/manual/todo-documentlist.html and open the manual test
2. Add a list item
3. Try to change the item's alignment

Result
The position of the list item doesn't change:

alignment.mp4

Additional info
Works good with other document list types and with legacy to-do lists

[niegowski]

The checkbox alignment extracted to the follow-up: #15035

Other issues were solved.

[charlttsie]

@niegowski
It will probably be a follow-up but one more thing is that Track Changes is not yet supported:

checkbox-tc.mp4