expose drag-and-drop clientX and clientY

[joeyh]

Available in all drag and drop events except for dragEnd.

Closes #189

[sjakobi]

This looks very reasonable to me. Can you give your approval, @HeinrichApfelmus?

[sjakobi]

CI may fail but this builds fine on my machine.

I think what's missing is that ClientXY should be exported?!

[HeinrichApfelmus]

Getting X and Y coordinates from a drag event seems useful.

However, there are two issues with this implementation:

• It turns out that clientX/Y is relative to the top-left corner of the display in the browser, but this need not coincide with the top-left corner of the HTML page if the use has scrolled the page vertically! We use

    var offset = $(this).offset();
    var x      = e.pageX - offset.left;
  

  for the mouse click events, but I do not know if we can obtain offset in the same way for drag events.

• The use of read and toString() is unsatisfactory. We can marshal JSON values, so it would be more appropriate to express the argument to fun as a JSON value and read its fields on the Haskell side.

I'm happy to look into this if no one else calls dibs, but it may take another day or two.