on-drop-file in window<%>
For platforms that support drag-and-drop, this method is called when
the user drags a file onto the window. Drag-and-drop must first be
enabled for the window with
accept-drop-files.
Under Mac OS, when the user double-clicks on a MrEd file or drags a
file onto the MrEd icon, the
on-drop-file method of the frontmost frame is called (if drag-and-drop is enabled
for that frame).