gimp_image_constructed(): connect to "notify" on GimpColorConfig
and call gimp_color_managed_profile_changed() on the image.

Remove the "notify" handler on GimpColorConfig from
gimpdisplayshell-handlers.c because it already connects to the
image's "profile-changed" signal.

Additionally, implement GimpColorManaged::profile_changed() in
GimpImage and invalidate the image and all layer previews. One step
closer to color managing image and layer previews.