Support multiple colors per sprite

[shikaan]

We may want to allow more than one color per sprite.

Things to consider:

• should it be completely free, allow palettes or a simple background/foreground/border would be enough?
• should we allow different colors for the border?

Once we have many colors we can start looking into dynamically generate also harder parts of the sprites. For example, we can generate shadow/light points (or allow users to define those points)