Colored onion skin + fixed a grid refresh issue

I think it was an "underrated must", drawing in-betweens is far easier
this way. Hope you like it!
- Added UI toggle buttons in the display option section.
- There are 4 options: normal, blue, red, bright indigo. No need to use
more colors because pure R & B are clearly different from common pencil
color = black/gray.
Merge pull request #210 from eVoLuTiVeMEDIA/master

Colored onion skin + fixed a grid refresh issue