usb4java/usb4java-javax#8 improvements

[SKART1]

#8 try to fix

[kayahr]

Thanks for your contribution but I have to decline this PR. Changing the visibility of classes or methods is not an option here because this breaks the compatibility to the javax.usb spec. An application which uses the javax-usb API of usb4java must also work with any other javax.usb implementation by just setting a different class name in the properties file. If the application relies on some internal usb4java-specific APIs then this can't work. That's why these classes are package private. And I'm not going to change this.

The problem with the wrong port numbers must be fixed somehow else. Currently usb4java-javax uses dynamically generated virtual port numbers. I'm not sure why I did this, maybe the first libusb version which I used didn't provide port numbers. Now I guess the code could be refactored to use the port numbers provided by libusb.