libgimpcolor, *: change GimpColorProfile to be a GObject
it used to be a typedef to gpointer and actually was a cmsHPROFILE. Change its API to be more "standard", remove the public close() function. The object caches both the cmsHPROFILE and the data/length ICC blob, so conversions between the two become obsolete (simply call get_lcms_profile() or get_icc_profile()). Adapt everything to the new API, but port it in a naive way for now, the code doesn't take advantage of the new possibilities yet (like refcounting).
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