Add color management options to the screenshot plug-in:

By default, it tries to tag the image with the monitor profile;
alternatively, there is an option to convert the image to sRGB.

This works mostly fine on *one* monitor given its profile is
configured correctly. With more than one monitor, funny things happen
depending on the platform and on what we are shooting (window, screen,
area). There are some FIXMEs left in the code.