[9] Review request for JDK-8160073: menu issues in case of two-display configuration

[Jim Graham]

JBS: https://bugs.openjdk.java.net/browse/JDK-8160073
webrev: http://cr.openjdk.java.net/~flar/JDK-8160073/webrev.00/

The details are in the comments in the bug report.  To summarize:

- send a notification when DPI changes to reevaluate scene.getXY()
- correctly scale event screen coordinates for Mouse/Scroll/Gesture events
   (code is copied from Menu event code that was already doing it)

			...jim