Allow to set additional manifest entries
It seems that this is already prepared in JavaFXJarTask.groovy but still commented out:
// not provided, fix later //createJarParams.manifestAttrs
It would be very usefull to have this feature in order to allow for things like adding splash screens in an easy way (via the manifest attribute SplashScreen-Image). This would be helpful since large javafx programs need quite some time to start.