[Mac] DRAG_DONE reports null transferMode when destination is external

When a DnD destination is in another app the transferMode reported by the DRAG_DONE DragEvent is always null even if the drop was accepted. This also means the event's isAccepted() call returns false. It doesn't matter if the drop is to the Finder or another JavaFX application.

Steps to reproduce
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- Launch two instances of the attached DragAndDrop program
- Click and drag in the upper-left green rectangle of one instance
- Finish the drop in one of the blue rectangles in the other instance

Expected result
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After the drop the source rectangle should report that the drag was accepted and what the drag operation was.

Actual result
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The source always reports the drag as rejected.