When setting up a notification on this attribute, RapaGUI will run your
event callback whenever one or more files have been dropped onto the
listview widget. The TriggerValue field of the message table will
be set to a table that contains a list of all files that have been
dropped onto the widget.
Additionally, the message table will contain the following two extra fields:
X:-
The x-position where the user has dropped the file(s). This will be relative
to the widget's left corner.
Y:-
The y-position where the user has dropped the file(s). This will be relative
to the widget's top corner.
See Notifications for details.
Note that Listview.DropFile is only ever triggered if Listview.DropTarget
has been set to True first.