In JDK 9 we enabled GTK3 support as opt in via a system property.
See http://openjdk.java.net/jeps/283
And https://bugs.openjdk.java.net/browse/JDK-8145547
However the default remained GTK2 due to concern over testing/bake time.
Now it has been almost 2 years since it was pushed to 9 without
any reports of major issues, and the next release (11) will be an LTS
which may need to run on systems which do not have GTK2.2 any more
we should switch the default, with the opt-in being to switch back to 2.2
This should be a simple minor code change, but with appropriate testing.
It probably still merits a release note and a CSR.
It should be coordinated with the same change in JavaFX.