[8u60] Review request: RT-39813: [Mac] JavaFX core dumps on Stage#close

[Morris Meyer]

Kevin, Chien and David,

Please review my fix for this Mac Stage focus close issue. The Mac OSX 
internals were forcing a callback event onto a non-focused window, and 
this preemptively asserts focus before allowing the event through.

         --morris

WEBREV - http://cr.openjdk.java.net/~morris/RT-39813.01/
JIRA - https://javafx-jira.kenai.com/browse/RT-39813