User interface suggestion : color lines and curves for gradients

[pierrelabreche]

Observations

• The Color line / Curve setting is located in the Settings menu, easy to miss
• Switching between Color line and Curve does not preserve the gradient mapping, when it would be possible to do so
• The control line setting "by luminance" is not obvious

Suggestions:

• Move the Color line / Curve setting as a dialog box control
• When switching between Color line and Curve, preserve the gradient mapping, if it is possible to do so
• The control line setting "by luminance" is not obvious, relabel as "Sort by luminance"