When dragging, there are several operations that may be performed. The copy
operation is used to indicate that the data being dragged will be copied from
its present location to the drop location. The move operation is used to
indicate that the data being dragged will be moved, and the link operation is
used to indicate that some form of relationship or connection will be created
between the source and drop locations.
You can specify which of the three operations are allowed for a drag source
by setting the effectAllowed property within a dragstart event listener.
Note that these values must be used exactly as defined below.
copy, move, or link
copy or link only
copy or move only
link or move only
no operation is permitted