Expose additional wall part coloring flags to ZScript

[grahf78]

Now that Side.SetSpecialColor has been fixed, I noticed that the method was changed to allow setting the useowncolors flag.

I'd like to suggest also exposing a way to manipulate the rest of the sidedef material color flags:

nogradient_<part> = <bool>;
flipgradient_<part> = <bool>;
clampgradient_<part> = <bool>;

This would allow modders to have full control over material colors over a map through ZScript.