[Cogl] RGB <-> HSL functions v2

[Damien Lespiau]

I fixed up a few things from the last series due to a late renaming of HLS to
HSL.

I also dropped the cogl_color_shade() patch, I don't think it gives a "shade"
as in the color mixed with black[1].

As for HSL Vs HSB(HSV is the same as HSB), I think having L is slighty better,
ie you are able to go from white to black by just tweaking L. FWIW, the CSS 3
Color module introduces HSL as well.

[1] http://en.wikipedia.org/wiki/Tints_and_shades