Which returns a GimpColorTransform to transform a GimpColorManaged's
pixels to a GtkWidget's color space, using a GimpColorConfig's
settings. This is *unfinished* API and in the end will enable simple
display color management for the app, libgimp and plug-ins.
Move some functions from libgimpwidgets/gimpwidgets.[ch]
and from app/widgets/gimpwidgets-utils.[ch]. Newly add
gimp_widget_get_color_profile() which is extracted from
modules/display-filter-lcms.c.
