Bug 737008 - Curve color always black

Don't hardcode "black" for the "Value" curve in GimpCurvesTool.
Instead, pass a NULL color to GimpCurveView, which is then interpreted
as the theme's text color (the curve's background is the theme's text
background color). We still hardcode pure red, green and blue for the
other curves.

(cherry picked from commit 90c967a39fc20203274f1f8a1948de3d537bf113)