t/ckeditor5-table/24: Implemented list component separators

[oleq]

Suggested merge commit message (convention)

Feature: Implemented list component separators (see ckeditor/ckeditor5#3178).

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Additional information

Part of ckeditor/ckeditor5-table#46.

[jodator]

I have doubts that it will be a valid HTML? The list items are <li> elements inside <ul> (right now they are not buttons). If we plan to switch list to set of to buttons/other elmeents then it's probably OK.

ps.: I've checked this in HTML validator:

Error: Element span not allowed as child of element ul in this context. (Suppressing further errors from this subtree.)

From line 8, column 308; to line 8, column 343

 left</li><span class="ck ck-list__separator"></span

Contexts in which element span may be used:
    Where phrasing content is expected.
Content model for element ul:
    Zero or more li and script-supporting elements.

[oleq]

You're right. I copy-pasted it from the toolbar component without much thinking. In fact... we can get rid of the ListSeparatorView and use ListItemView with an additional CSS class instead.

[oleq]

I fixed this issue. It turned out we cannot use the ListItemView for that purpose because it implements the focusable interface and the separator must be transparent for the keyboard navigation.

[jodator]

I find this clean to have ListSeparatorView instead of reausing ListItemView so it gonna be OK.