Whether drag was canceled with this event. Used to differentiate between
a legitimate drag END that can result in an action and a drag END which is
a result of a drag cancelation. For now it can happen 1) with drag END
event on FireFox when user drags the mouse out of the window, 2) with
drag END event on IE7 which is generated on MOUSEMOVE event when user
moves the mouse into the document after the mouse button has been
released, 3) when TOUCHCANCEL is raised instead of TOUCHEND (on touch
events).