app, libgimp*: more GeglColor's space invasion.

- New function gimp_cairo_set_source_color() which is meant to replace
  gimp_cairo_set_source_rgb(a?)() eventually. This new function sets the Cairo
  source color, using the target monitor's profile of the widget where the Cairo
  surface is meant to be drawn on. It also uses the color management settings
  (such as whether a custom profile was set, instead of using system profile, or
  also simply whether color management was disabled at all). It doesn't
  soft-proof the color yet.
- Padding and out-of-gamut colors drawing now use the new
  gimp_cairo_set_source_color(). These don't need any soft-proofing anyway.
- Out-of-gamut color property in GimpColorConfig is now a GeglColor property.

libgimpwidgets/gimpwidgetsutils.h

@@ -68,6 +68,15 @@ GimpColorTransform * gimp_widget_get_color_transform (GtkWidget         *widget,
 void                 gimp_widget_set_native_handle    (GtkWidget        *widget,
                                                        GBytes          **handle);
 
+
+/* Internal use */
+
+G_GNUC_INTERNAL void _gimp_widget_get_profiles        (GtkWidget         *widget,
+                                                       GimpColorConfig   *config,
+                                                       GimpColorProfile **proof_profile,
+                                                       GimpColorProfile **dest_profile);
+
+
 G_END_DECLS
 
 #endif /* __GIMP_WIDGETS_UTILS_H__ */