Implement editing part for List Styles feature

[pomek]

Part of #1031.

[pomek]

As for now, I have a basic implementation of the feature.

For a bulleted list, you can define a type of marker that will be used when rendering the list.

The same applies for numbered lists:

And even for nested and mixed lists:

However, it's the beginning of problems since the feature must be integrated with other plugins.

Indenting lists

When creating at some point a nested list, it should not inherit the list style attribute/value from the parent list.

■ List item 1.
■ List item 2.[]
■ List item 3.

editor.execute( 'indentList' );

■ List item 1.
    ○ List item 2.[]
■ List item 3.

However, the deeper structures should not be modified:

■ List item 1.
■ List item 2.[]
    ⁃ Nested item 1.
    ⁃ Nested item 2.
■ List item 3.

editor.execute( 'indentList' );

■ List item 1.
    ○ List item 2.[]
        ⁃ Nested item 1.
        ⁃ Nested item 2.
■ List item 3.

Also, it should work correctly when between list items, other nested lists are placed:

■ List item 1.
    ○ List item 2.
        ⁃ Nested item 1.
        ⁃ Nested item 2.
■ List item 3.[]
■ List item 4.

editor.execute( 'indentList' );

■ List item 1.
    ○ List item 2.
        ⁃ Nested item 1.
        ⁃ Nested item 2.
    ○ List item 3.[]
■ List item 4.

The attribute was restored to the default value. However, if List item 2 has modified the list style, List item 3 should copy the list style from 2.

■ List item 1.
    ♥ List item 2.
        ⁃ Nested item 1.
        ⁃ Nested item 2.
■ List item 3.[]
■ List item 4.

editor.execute( 'indentList' );

■ List item 1.
    ♥ List item 2.
        ⁃ Nested item 1.
        ⁃ Nested item 2.
    ♥ List item 3.[]
■ List item 4.

Outdenting lists

More or less it should revert changes made by the indentList feature.

A simple scenario:

■ List item 1.
    ○ List item 2.[]
■ List item 3.

editor.execute( 'outdentList' );

■ List item 1.
■ List item 2.[]
■ List item 3.

Don't modify deeper structures:

■ List item 1.
    ○ List item 2.[]
        ⁃ Nested item 1.
        ⁃ Nested item 2.
■ List item 3.

editor.execute( 'outdentList' );

■ List item 1.
■ List item 2.[]
    ⁃ Nested item 1.
    ⁃ Nested item 2.
■ List item 3.

And copy the value for the listStyle attribute from an item that is on the same (indent) level:

■ List item 1.
    ○ List item 2.
        ⁃ Nested item 1.
        ⁃ Nested item 2.
    ○ List item 3.[]
■ List item 4.

editor.execute( 'outdentList' );

■ List item 1.
    ○ List item 2.
        ⁃ Nested item 1.
        ⁃ Nested item 2.
■ List item 3.[]
■ List item 4.

However, the outdent feature can replace the listItem element with the paragraph element. No action is required in such a situation.

Merging a block to a list

A case:

Paragraph[]
■ List item 1.
■ List item 2.
■ List item 3.

editor.execute( 'bulletedList' )

The block item (paragraph) will be replaced with the listItem element. And the question is, should it copy the lists styles? In other words, should it be like:

* Paragraph[]
■ List item 1.
■ List item 2.
■ List item 3.

or

■ Paragraph[]
■ List item 1.
■ List item 2.
■ List item 3.

The later seems to be correct. Fortunately, the first item on the list cannot be nested, so it shouldn't be too hard to implement.

In all scenarios, I was considering operations between the same type of lists (all nested lists are bulleted or numbered, but never both at the same time). What in case of mixed lists? As for now, I am treating them as separate elements.

[pomek]

Another case:

What should happen after removing the paragraph? Should the list be merged and all rendered with circle marker or treat those as separate lists?

[pomek]

Another case:

After outdenting, items 2.1 and 2.2 should inherit list styles from the list item 3.

[pomek]

Maybe listening to editor#deleteContent() with higher priority could help. We could detect whether the list will be merged and adjust the listStyle attribute for the merged (second) list.

[pomek]

All issues I've spotted are extracted into a separate issue: #7879