callback when user has canceled the popup by
1. hitting ESC or
2. by using the popup widget's proprietary cancel mechanism (like a cancel button in a dialog);
i.e. whenever popupWidget.onCancel() is called, args.onCancel is called
|| String
When the around parameter is specified, orient should be an
ordered list of tuples of the form (around-node-corner, popup-node-corner).
dijit.popup.open() tries to position the popup according to each tuple in the list, in order,
until the popup appears fully within the viewport.