Warn if JavaFX is not supported on current JVM for plugins that use it

[jpsacha]

By default ImageJ is using Java 8 that may not support running current JavaFX. Users are getting cryptic exception errors when attempting to run Color Calibrator or other plugins that require JavaFX.

Provide better diagnostic information for the user if there is an issue with JavaFX + help how to upgrade Java to resolve the issue

[Yvona27]

Hello,

I am an undergraduate studying biology. I am finding tools to do color correction on butterfly photos before wing pattern analysis. The discovery of this plugin is very helpful but I also countered the Java.lang.UnsupportedClassVersionError. I would like to kindly ask would you still update this plugin and whether this problem can be solved by other ways at the moment. Thank you for all the effort developing this plugin, it can be very handy for ecologists!

Kind regards