Draft: Issue #9530: Add Preferred CMYK option from GimpColorConfig to status bar and menu soft-proofing.

This patch attempts to resolve issue #9530. It adds the Preferred CMYK profile from GimpColorConfig as the second option in the menu soft proofing dialogue and the status bar soft-proofing pop-over menu if set.

Unlike the various dialogues where Preferred Profiles are used, the status bar combobox is persistent. Therefore, the combo box model is cleared out and rebuilt every time the preferences are changed.

A new internal function gimp_statusbar_build_profile_combo () is also added to consolidate the building/rebuilding code for the soft-proofing combobox box.

Edit: This causes a memory leak that I need to fix!