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

[Kevin Rushforth]

On Wed, 11 Dec 2019 00:21:20 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

>>> 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.

With your latest patch, which reverts the changes to `GtkDnDClipboard.java`, I now get the DRAG_DONE event, but the following warning printed to the console in the case of a `DRAG_DONE` event with a null transfer mode:

GOT A dragExit when dndGesture is null!

This is coming from [Scene.java#L2925](https://github.com/openjdk/jfx/blob/master/modules/javafx.graphics/src/main/java/javafx/scene/Scene.java#L2925).

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

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