Use user preference + monitor placement to control image window color management

Surprisingly little code required to do such a monumental task!  Thank
goodness for advance planning...

It's quite neat to set up an arbitrary color profile for a secondary
monitor, then move the main window around and see how images redraw
themselves using the profile of whatever monitor intersects them the
most.

Unfortunately, now that I've done this, I'm quite dissatisfied with
other software, as it does not offer such a helpful feature.  :(