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

...when a color profile is active

Add GimpPickable::pixel_to_srgb() which puts a picked raw image
pixel into a GimpRGB. Default to gimp_rgba_set_pixel() but implement
pixel_to_srgb() in GimpLayer, GimpProjection and GimpImage and
run the pixel through gimp_image_color_profile_pixel_to_srgb().