Hide the existence of color value mapping

[heinermann]

Users are not interested in the color values of tiles. Hide this value and generate ones for new brushes on the fly.

[cstamford]

If I recall correctly, a new RGBA value for brushes is auto-generated when using the new brush wizard already.

[heinermann]

This was meant as more of a removal of the colour map and brushes. I'll see if I can outline my ideas for this after I fix the current bugs.