Colored onionskin completion

As some beta-testings and feedbacks are required, here are some tips for colored onionskins usage:

1. In the display settings section, there are two buttons to select from blue, red and purple(two buttons down).
2. If "prev" or "next" onionskins are selected (not both), any color from the 3 will be used.
3. If both "prev" and "next" onionskins are selected, "previous frames" will be shown in "red" and "next frames" will be shown in "blue".

You'd better try it yourself, I'm sure you will understand how easy and intuitive it is.