Add softproof profile to GimpImage (Part 1)

This patch attempts to implement the first two steps of #8148 (closed). It does the following things:

• Adds "simulation_profile" to GimpImagePrivate's data
• Creates two plug-in accessible functions to get and set this new profile:
  • gimp_image_get_simulation_profile () and gimp_image_set_simulation_profile ()
• Changes the soft-proofing preview code to use this profile instead.
• Updates the menus and image-actions/image-commands to let users change this profile from the Image menu GUI
  • Duplicate soft proofing functions are removed from the View menu GUI's Color Management section
• A new signal, "simulation-profile-changed" is now emitted from GimpImage via its GimpColorManagedInterface. This has been tested by modifying the GimpColorFrame code in !589 (merged) to listen for this signal on the image instead of GimpDisplayShell.

Feedback appreciated!