Ckeditor5: Support for drag&drop of blocks

[Reinmar]

📝 Provide a description of the new feature

Extracted from #2664. Scenario C and possibly D.

---

If you'd like to see this feature implemented, add a 👍 reaction to this post.

[Reinmar]

We're hesitant between going with C (widgets only) only or C and D (all blocks).

We'd probably add some "handle" next to each block. Maybe, Notion-style. The current selection handle for widgets would become obsolete.

On click, that handle would select that block.

On drag, it would start reorganizing blocks.

[neongreen]

I think a Notion-style handle for all blocks is strictly better — for users, at least.

Do you also plan to allow dragging images by clicking anywhere on the image (not just the drag handle)?

[neongreen]

Another thing to keep in mind is lists — if you consider each list a single block element, it would make sense to make an exception here and add drag handles to list elements instead of one-per-list.