Add tracking function, add arrow to indicate direction.

[wzw1105]

1. The angle of box is converted to the range of [-pi, pi], positive in counterclockwise direction.
2. Add a red arrow to each box to indicate the direction for better visualization.
3. Newly add 'Track' button to track the boxes and labels in previous image to next image, then we just need to fine tuning the box rather than redraw the boxes. The function is useful when you label the images which are consecutively shooted.

[mmmmadv]

good job