Color ID for cells

[tehtmi]

Quoting ​http://forum.chaosforge.org/index.php/topic,4422.0.html:

Sometimes there are several cells (tiles) that are intended to look alike to the player. The best example is probaly "wall" and "wall2", but also "pwall" (or p versions of anything). By changing color bindings, the player can (intentionally or unintentionally) distinguish between these tiles.

I suggest adding a "color_id" property to cells in the style of levers so that e.g. wall2 will use the color binding for wall.

Modders might also find this useful for e.g. hiding secret doors.