Position drag element relative to mousedown on original draggable

[jmgregory]

Tested on Safari, Firefox, and Chrome on a Mac. Needs testing (at minimum) on IE / Edge in Windows.

This only applies to the automatically generated clone element, not custom template elements.

Original issue can be found here: #34

[mwoc]

I'm taking a look at it in the examples, and kind of like the proposed behaviour.

When looking at the 'Using a custom drag element' example, I can see that that drag element still attaches to the right of the mouse cursor, which I think makes sense.

It makes the the 'sortable' example look slightly messy though, as the names overlap each other when you start dragging - but for many others it looks much nicer.