Allow ProgressBarBuilder.setETAFunction to be used externally

[deejgregor]

ProgressState is package-private and setETAFunction's BiFunction includes ProgressState, preventing setETAFunction from being used outside of the package.

Reproducer:

import java.util.Optional;

import me.tongfei.progressbar.ProgressBarBuilder;

public class SetETAReproducer {
    public static void main(String[] argv) {
        ProgressBarBuilder pbb = new ProgressBarBuilder();
        pbb.setETAFunction((a, b) -> Optional.empty());
    }
}

Run this on the latest released version 0.9.4:

$ javac -cp $HOME/.m2/repository/me/tongfei/progressbar/0.9.4/progressbar-0.9.4.jar SetETAReproducer.java
error: ProgressState is not public in me.tongfei.progressbar; cannot be accessed from outside package
SetETAReproducer.java:8: error: ProgressState is not public in me.tongfei.progressbar; cannot be accessed from outside package
        pbb.setETAFunction((a, b) -> Optional.empty());
                           ^
2 errors