Activate color management for tool preview windows

AutoRedraw proved especially problematic for preview windows.
Explicitly setting the StretchBlt mode - a necessity for shrinking the
image to the preview space - seems to deactivate any color management,
for reasons I can't possibly fathom.

Thus, my choices were to move some color management code into the layer
class, which I didn't want to do, or to switch off AutoRedraw and handle
color management inside the preview user control itself.  I went with
the latter.

This task also demonstrated the necessity of color managing color
selection boxes - if this is not done, and the user selects a color from
the image, the matching color box may appear to display a different
color!