Added new color toning method "L*a*b* regions"

Allows to specify various "regions" of the image with masks, and to correct
for hue, saturation and lightness.

Inspired by the existing L*a*b* grid (in turn taken from darktable)

rtgui/paramsedited.h

@@ -178,6 +178,8 @@ public:
     bool labgridBLow;
     bool labgridAHigh;
     bool labgridBHigh;
+    bool labregions;
+    bool labregionsShowMask;
 };
 
 class SharpenEdgeParamsEdited