[11] Review request: 8198654: Switch FX's default GTK version to 3

[Kevin Rushforth]

Phil,

Please review the FX change to use GTK 3 by default, as you recently did 
for AWT:

https://bugs.openjdk.java.net/browse/JDK-8198654
http://cr.openjdk.java.net/~kcr/8198654/webrev.00/

I have run a full test, including all robot tests, on Ubuntu 16.04 using 
two different JDKs -- one with your AWT fix and one without. No new 
issues were discovered.

Thanks.

-- Kevin