[8u] RFR: backport of JDK-8214765, All TrayIcon MessageType icons does not show up with gtk3 option set

[Simon Tooke]

This is a straightforward backport to use new filenames for icons if the
version of gtk is >= 3.10.

Issue: https://bugs.openjdk.java.net/browse/JDK-8214765

Webrev: http://cr.openjdk.java.net/~stooke/webrevs/jdk-8214765/

Is it okay?

-Simon