RFR: 8325591: [Mac] DRAG_DONE reports null transferMode when destination is external [v2]

[Martin Fox]

> At the end of a drag operation the Mac Glass code sends out a DRAG_DONE event using the operation mask tracked in the GlassDragSource to determine the final transfer mode. That mask is only updated when a window in the JavaFX app is the drop destination. If the drag moves to an external destination the mask is set to NONE. If the drag terminates in the external destination that NONE forms the basis of the transfer mode sent via the DRAG_DONE event.
> 
> At the very end of the drag the OS calls the NSDraggingSource (GlassDraggingSource) with the final drag operation. This PR issues the DRAG_DONE from that callback so it can get the final transfer mode correct for both internal and external destinations.

Martin Fox has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Added manual test that covers all the DnD basics
 - Merge remote-tracking branch 'upstream/master' into macdropop
 - Using OS to determine DRAG_DONE operation

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1371/files
  - new: https://git.openjdk.org/jfx/pull/1371/files/4f088cc3..c0bbbf5c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1371&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1371&range=00-01

  Stats: 32641 lines in 1003 files changed: 17817 ins; 6536 del; 8288 mod
  Patch: https://git.openjdk.org/jfx/pull/1371.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1371/head:pull/1371

PR: https://git.openjdk.org/jfx/pull/1371