Which function to call to handle a drop of that type.
If the type for the drop is not present, or the function is nil,
the drop is rejected. The function takes three arguments, WINDOW, ACTION
and DATA. WINDOW is where the drop occurred, ACTION is the action for
this drop (copy, move, link, private or ask) as determined by a previous
call to `x-dnd-test-function'. DATA is the drop data.
The function shall return the action used (copy, move, link or private)
if drop is successful, nil if not.