add HID mapping for Hercules DJ Console MP3

Add mapping provided by a user from this Bug
https://bugs.launchpad.net/mixxx/+bug/1232822
Merge pull request #336 from kain88-de/dj-control-mp3-midi

add HID mapping for Hercules DJ Console MP3
Merge remote-tracking branch 'mixxxdj/pr/380'
Check for NULL tracks on mouse release in WStarRating.
Merge remote-tracking branch 'mixxxdj/pr/380'
resolving conflict