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?