Better hint value for step listener

[Former user]

It could be helpful to have more kinds of hint instead of "Evaluation loop". e.g. For delta calculation while solving a quadratic equation

[Axel Kramer]

I've prepared the IStepListener for better managing of the current hints in this commit: https://bitbucket.org/axelclk/symja_android_library/commits/e52bca61db66c8c5bcc3a86e69d84f154b3494f0

Do you have an example expression where the step listener should create better hint messages?