Bug 748749 - picked colors don't match image colors...

...when a color profile is active

This commit doesn't fix anything, but it prepares the code to do the
right thing:

It passes the actual raw image pixels through the entire color picking
mechanism to the widgets which display colors, particularly
GimpColorFrame.

This is needed for GimpColorFrame's "Pixel" mode (as opposed to its
RGB, HSV etc. modes) which is supposed to show the raw pixel values
from the image.

Before this commit, it was recreating the raw pixel values from the
GimpRGB value it knows, which will become impossible when we correctly
pick color managed GimpRGB values soon.