color calibration module - improve white balance picker handling

[rgr59]

The white balance picker of the color calibration module can lead to strange results if the user is not aware that he has set a target color different from neutral. This is well documented, but every so often people (me included) stumble over this. There is an "active" appended to the label of the "area color mapping" collapsible section if the values differ from neutral since some time now, but it can easily be overlooked.

This could be improved by a change in the color calibration GUI:

• the current picker always and only sets the white balance using neutral values
• in the "area color mapping" collapsible section, a second picker is added that, depending on "area mode", picks a color from a source image or applies the color to target images.

Of course this would be a change of the GUI and I don't know if this is wanted. But it could help avoid mistakes: a user wanting to transfer colors between images must expand the section to get hold of the picker, so he is automatically aware of this functionality and also sees the currently configured target values.

[elstoc]

I suggested this same UI when the area color mapping feature was introduced and the original author of the functionality strongly objected (though he's left the project now). I very much support this feature request and IMO something similar should be done in the exposure module (in which case the main picker should set the exposure to 50%).