Inclusion of :drag-over in CSS4 UI

Firefox has had the :-moz-drag-over[1] pseudo-class for sometime which
allows you to style an element when the drag over event is called on it.

I think this would be a useful inclusion to be considered for the CSS4 UI
module, albeit perhaps with a more descriptive name.

[1] https://developer.mozilla.org/en/CSS/:-moz-drag-over

-Ryan Seddon