RFR: 8225571: Port DND source to use GTK instead of GDK

[Kevin Rushforth]

On Tue, 10 Dec 2019 23:53:27 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>> @kevinrushforth I changed that behaviour. Since the drag does not complete i find it counter-intuitive to send the DRAG_DONE event.
>> 
>> Does it send the event on other platforms? I can test on Windows.
>> 
>> Will look into the assertion error - it does not happen on 18.04.
> 
>> Does it send the event on other platforms? I can test on Windows.
> 
> Yes, it always sends the `DRAG_DONE` event on all platforms today (I tested it on Mac, Windows, and Linux).
> 
>> Will look into the assertion error - it does not happen on 18.04.
> 
> It doesn't happen on 19.10 either. Only 16.04 as far as I have tested. I'll also test on Oracle Linux 7.x.

I don't get an assertion error on Oracle Linux 7.7 -- so far only on Ubuntu 16.04. I will resume reviewing once you restore the `DRAG_DONE` behavior.

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PR: https://git.openjdk.java.net/jfx/pull/4