[FEATURE] Implemented a composite RGB curve for Curves filter

Now the curves filter provides two ways of adjusting image brightness:

1) Adjusting a grouped RGB curve
2) Adjusting L channel of Lab separation of the image using 'Lightness'
   curve

http://nonaynever.ru/kwiki/krita/published/L_vs_RGB_curve_comparison

Ref T352
CC:kimageshop@kde.org