t/ckeditor5/416b: (WIP) Implemented dynamic items grouping for overflowed toolbars

[oleq]

Suggested merge commit message (convention)

Feature: Implemented dynamic items grouping for overflowed toolbars (see ckeditor/ckeditor5#416).

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Additional information

• Requires:

  • https://github.com/ckeditor/ckeditor5-core/compare/t/ckeditor5/416b?expand=1
  • https://github.com/ckeditor/ckeditor5-editor-classic/compare/t/ckeditor5/416b?expand=1

• This is WIP (PoC).

• Things missing:

  • Tests,
  • Docs,
  • I don't like the way this feature is enabled (after editor#ready, see the code in editor-classic),
  • Support for toolbar separators (e.g. they should not be displayed when the first child in the dropdown)
  • (When t/416: Improved responsiveness of the forms in narrow viewports ckeditor5#1299 is merged): The toolbar in the dropdown should wrap after N-items or pixels. On very narrow screens (like iPhone SE), the grouped items toolbar can go beyond the edge of the viewport. I researched 4xN CSS grid and it looked nice too.
  • Feature should be enabled in DocumentEditor by default.