[DND] FX sometimes forgets to invoke Node.onDragDetected() on Mac

Working on Scene Builder, I noticed that sometimes Node.onDragDetected() is not invoked
as expected. So far, this issue happened only on Mac.

I did a sample project to reproduce the issue (see DragDetectedTest.zip).

To reproduce the pb:

1) Unzip and open DragDetectedTest project with NB (project needs a "JDK 1.8" platform)
2) Run
    => a blue window is displayed
3) Press the mouse over the blue area
    => mousePressedListener() is called
4) Drag the mouse
    => dragDetectedListener() is called
    => dragOverListener() is called several times
5) Release *AND KEEP THE MOUSE IMMOBILE*
    => dragDroppedListener() is called
    => dragExitedListener() is called
    => dragDoneListener() is called
    => OK
6) *WITHOUT MOVING THE MOUSE*, press again the blue area
    => mousePressedListener() is called
7) Drag the mouse
    => dragDetectedListener() is not invoked : BUG
8) Release the mouse
    => mouseReleaseListener() is called
    => Drag & drop session has not been started : BUG

Note that, if you move the mouse between steps #5 and #6,
then it works ok : drag & drop session is started and managed correctly.
So the key condition is : release and press again without moving the mouse.