Function: x-dnd-drop-data

Drop one data item onto a frame.
EVENT is the client message for the drop, FRAME is the frame the drop
occurred on. WINDOW is the window of FRAME where the drop happened.
DATA is the data received from the source, and type is the type for DATA,
see `x-dnd-types-alist').

Returns the action used (move, copy, link, private) if drop was successful, nil if not.