public void setSettingsHandler(BiFunction<AppSettings,Boolean, Boolean> handler){
        settingsHandler = handler;
    }
Set a function to handler app settings. 
The default implementation shows a settings dialog if available.
@param handler handler function that accepts as argument an instance of AppSettings 
to transform and a boolean with the value of true if the settings are expected to be loaded from 
the user registry. The handler function returns false if the configuration is interrupted (eg.the the dialog was closed)
or true otherwise.
jme3-core/src/main/java/com/jme3/system/JmeSystemDelegate.java