[FXDnD]: in removeDropTarget the isDropTargetListenerInstalled field isn't reset

If you set the dropTarget of a SwingNode's content JComponent to null once, the drag event handler are removed but the mentioned field isn't reset.
If you want to use a new DropTarget later, FXDnD.addDropTarget won't register the event handler for the drag events and the DropTarget doesn't receive any events.