Improve notebook cell drag and drop

[kasbah]

• Fix fiddly-ness around where the mouse is when dropped
• Use the notebook cell itself as the preview
• Allow dropping a cell back where it was (most intuitive way to cancel the drag and drop)

[epatters]

Thanks Kaspar, this is much appreciated! @quffaro and I have both spent time on drag and drop and it's amazing how fiddly this stuff is.