missing re-trace capability

[Rockfella]

Great job making this possible! One of the biggest functions I am missing is the possibility to re-trace, like move back to the frame where it looses it´s trace and then adjust the positioning of the tracker.

I am trying this out on sports applications like movement analysis. I have modified the code to change the colors according to magnitude of translation which looks pretty cool, in addition to that combining sensor data such as EMG or Accelerometer to color the traces.