RFR: 8233955: VM crashes if more than one file are added to ClipboardContent via drag and drop [v3]

[Lukasz Kostyra]

> Crashes started happening due to macOS DnD API change from macOS 10.14 onwards. 10.14 incrodues some [DnD constrains](https://developer.apple.com/documentation/macos-release-notes/appkit-release-notes-for-macos-10_14#Drag-and-Drop) which our DnD code happened to trigger on some occasions.
> 
> Our code used deprecated `dragImage` API which since 10.14 had new requirement - each NSPasteboard item had to have a corresponding drag image. Not meeting this constraint raised an exception, which crashed the app. Since there was no possibility to add more drag images to `dragImage` API (it only took one NSImage as parameter) the code had to be rewritten - as such I upgraded it to new DnD API utilizing NSDraggingSession and related protocols.
> 
> One side-effect of new DnD API is that it now modifies NSPasteboard for us - previous behavior was more "separated" from user code perspective (write items to pasteboard -> initiate a drag via `dragImage`). Manually updating NSPasteboard before calling `beginDraggingSessionWithItems` raised another exception related to NSPasteboard already having DnD-ed elements inside it. Some system tests, however, relied on that behavior and writing to NSPasteboard (`MacPasteboardShim.java` used in some tests creates a `Clipboard.DND` for test purposes). Since this path is in tests I assumed this behavior should stay and tried to make it as close to working as possible. Tests (including those using `MacPasteboardShim`) pass after my changes.
> 
> Additionally, added a new manual test based on `DndTest.java` test which creates two temporary files and allows for testing faulty behavior.

Lukasz Kostyra has updated the pull request incrementally with two additional commits since the last revision:

 - GlassViewDelegate: Address review issues
   
   * Removed draggingSource pointer
   * Removed some leftover comments
   * Set delegate to nil
 - Revert "GlassViewDelegate: Return to deprecated *PboardType symbols"
   
   This reverts commit fa42b1b13755dec05d77b3865ed657e954854286.
   
   fa42b1b13 is not needed anymore - JFX will target macOS 11 soon.

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1089/files
  - new: https://git.openjdk.org/jfx/pull/1089/files/fa42b1b1..a8b5b3de

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1089&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1089&range=01-02

  Stats: 27 lines in 2 files changed: 13 ins; 9 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1089.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1089/head:pull/1089

PR: https://git.openjdk.org/jfx/pull/1089