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

[Thiago Milczarek Sayao]

https://bugs.openjdk.java.net/browse/JDK-8225571

To run tests (on the root of the source tree):
/gradlew apps
java @build/run.args -cp apps/toys/DragDrop/dist/DragDrop.jar dragdrop.DragDropWithControls
java @build/run.args -cp apps/toys/DragDrop/dist/DragDrop.jar dragdrop.DragDropColor

java -Djdk.gtk.version=2 @build/run.args -cp apps/toys/DragDrop/dist/DragDrop.jar dragdrop.DragDropWithControls
java -Djdk.gtk.version=2 @build/run.args -cp apps/toys/DragDrop/dist/DragDrop.jar dragdrop.DragDropColor

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Commits:
 - 3d650b2b: Gtk DND port for Linux

Changes: https://git.openjdk.java.net/jfx/pull/4/files
 Webrev: https://webrevs.openjdk.java.net/jfx/4/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225571
  Stats: 669 lines in 5 files changed: 81 ins; 442 del; 146 mod
  Patch: https://git.openjdk.java.net/jfx/pull/4.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/4/head:pull/4

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