More advanced onion skinning

[blurymind]

Improve onion skinning

• color code past frames in red and future frames in blue. (specify colors in preferences)
• Ability to onion skin only keyframes and not inbetweens to me onion skinning the inbetweens is useful only when you do the spacing of movement. For posing, it is key to tell the onion skin to show the previous/next (1-5 or 6 at least) keyframes without the tweens
• later on - Ability to set specific keyframes to be onion skinned and in what color. See onion skin editor in tvpaint for reference