drop target notifications are sent out of order during DnD

This issue was found while working on the following JavaFX bug:

https://javafx-jira.kenai.com/browse/RT-28347

What I observe is that when a JFXPanel object acts as a drop target, dragEnter(), dragOver(), and dragExit() notifications can be sent to its DropTargetListener in a wrong order. For example:

drag enter
drag over
drag exit
drag over

or

drag enter
drag enter

To reproduce the bug, run the following JavaFX/Swing test and drag-n-drop from and to the FX scene:

            JFrame f = new JFrame("F");
            f.setBounds(400, 100, 320, 240);

            JFXPanel p1 = new JFXPanel();
            Platform.runLater(() -> {
                Text t = new Text("Drag from here");
                t.setStyle("-fx-font-size: 32px");
                t.setOnDragDetected(new EventHandler<MouseEvent>() {
                    @Override
                    public void handle(MouseEvent event) {
                        Dragboard db = t.startDragAndDrop(TransferMode.COPY);
                        ClipboardContent content = new ClipboardContent();
                        content.putString("FX TEXT");
                        db.setContent(content);
                        event.consume();
                    }
                });
                t.setOnDragDone(new EventHandler<DragEvent>() {
                    @Override
                    public void handle(DragEvent event) {
                        t.setText("Drag done");
                    }
                });
                Text t2 = new Text("Drop here");
                t2.setStyle("-fx-font-size: 32px");
                t2.setOnDragEntered(new EventHandler<DragEvent>() {
                    @Override
                    public void handle(DragEvent event) {
                        System.out.println(event.getDragboard().getContentTypes());
                        event.acceptTransferModes(TransferMode.ANY);
                        event.consume();
                    }
                });
                t2.setOnDragOver((event) -> {
                    event.acceptTransferModes(TransferMode.ANY);
                    event.consume();
                });
                t2.setOnDragDropped(new EventHandler<DragEvent>() {
                    @Override
                    public void handle(DragEvent event) {
                        t2.setText(event.getDragboard().getString());
                        event.setDropCompleted(true);
                        event.consume();
                    }
                });
                Scene s = new Scene(new FlowPane(t, t2), Color.RED);
                p1.setScene(s);
            });
            f.add(p1);

            f.setVisible(true);

If you see exceptions (or assertion errors, when run with -ea), the bug is reproduced. These exceptions are observed on Mac, but not on Windows, both before and after the fix for RT-28347.