to its dnd_motion() method; its
dnd_leave() method has not been called since the last call to
dnd_enter().  The target is notified of the drop by a call to its
method dnd_commit(source, event).

If no final target object is selected, and there was an old target
object, its dnd_leave(source, event) method is called to complete the
dnd sequence.

Finally, the source object is notified that the drag-and-drop process
is over, by a call to source.dnd_end(target, event), specifying either
the selected target object, or None if no target object was selected.
The source object can use this to implement the commit action; this is
sometimes simpler than to do it in the target's dnd_commit().  The
target's dnd_commit() method could then simply be aliased to
dnd_leave().

At any time during a dnd sequence, the application can cancel the
sequence by calling the cancel() method on the object returned by
dnd_start().  This will call dnd_leave() if a target is currently
active; it will never call dnd_commit().

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