refactoring of gfx.Color to accomodate HSV better

The Color class now keeps track of RGB and HSV in preparation of better HSV use,
e.g. HSV based gradients. The interface has been extended, and parts of the old
interface have been deprecated (by means of in-source comments, not yet as
messages when using it); this applies to both C++ and Python. Instead of
gfx.Color() use the factory functions gfx.RGB(r,g,b), gfx.RGBA(r,g,b,a),
gfx.HSV(h,s,v) and gfx.HSVA(h,s,v,a). See also gfx/color.hh for in-source doc