Add "gboolean color_managed" and "GFile *color_profile" to
GimpTemplate. Add a toggle and profile combo to GimpTemplateEditor.
Honor the new template properties in gimp_image_new_from_template().

Using a GFile property instead of a GIMP_TYPE_CONFIG_PATH is
preliminary, see the previous commit. I'd like to use GFile more
directly when dealing with config files, this is for testing that.