Color Schemes

I’m adding a few minor changes to the engine interface and color.h.
The basic idea is to allow

ghutchis: so the engine interface will change slightly: two new
methods to get a private Color object
ghutchis: and set one.
dcurtis: well, the only thing i’m thinking is that shouldn’t color
be a property of each engine?
dcurtis: like, what is a color scheme for one engine won’t be for
dcurtis: or you want them to share color schemes?
ghutchis: yes, a color scheme for one engine could be different from
ghutchis: better yet… that we can have color schemes.
ghutchis: right now, there’s just one (color by element)

The code still feels a bit rough. I thought about offering a color
gradient feature (e.g., to indicate electronic charge or something)
but decided not to. I changed the engines to use the color maps for
transparent selections, but maybe it’s better to add something like

Now, I think we should be able to add a few different types of color

Suggestions, cleanups, flames, are most welcome.