Expose simple way to trigger lineutils-based DnD

[Piotrek Koszuliński]

(Note: lineutils-based DnD is the mechanism used while dragging block widgets)

Use cases:

1. When user starts dragging an inline image (or image2) I would like to be able to enable line-utils-based DnD. I imagine that I can listen on editor#dragstart, check what's dragged, and use some editor method, to do what I want. Of course this rises questions like - what with empty blocks left after dragging an image from them - this would be handled by the developer which implements such feature or by CKEditor. We could e.g. make it possible to alter dragRange (it may even be possible now) so a developer would be able to select whole paragraph.

2. Dragging some stuff from outside of the editor. E.g. there could be a list of products and dragging it into editor should be done with line utils.

3. Dragging table rows with line utils?

4. Must it be drag and drop? Maybe we can expose this feature for more user actions. You click a widget, choose "move" and while your moving your cursor above the editor, then line utils appear with an icon like magicline. Clicking this icon moves the element there.
   • With some more UX-related improvements we could enable magicline and lineutils-based DnD on touch devices.