title: drag and drop a node into another node
body: <p>I'm trying to learn JavaFX by implementing a chess game.
  My problem is I have no idea how to move a piece by dragging and dropping .
  In my code I have a 2D array of label in which label contains an ImageView.
  I want to know how to determine another piece (node) under the dragged piece.
  Thanks in advance.</p>