app, libgimp*, pdb, plug-ins: GimpContext is now using only GeglColor.

- app: gimp_context_get_(foreground|background)() are now returning a GeglColor.
- libgimp: PDB functions named similarly in libgimp are returning a newly
  allocated GeglColor too.
- A few other PDB functions (the ones using these functions) were updated and
  their signature changed to use GeglColor too, when relevant. Plug-ins which
  use any of the changed libgimp functions were fixed.
- GimpContext: signals "(foreground|background)-changed" are now passing a
  GeglColor.
- libgimpconfig: new macro GIMP_CONFIG_PROP_COLOR using gegl_param_spec_color().
- GimpContext: properties "foreground" and "background" are now GeglParamColor
  properties.
- app: All code interacting with GimpContext objects were updated to receive a
  GeglColor (that they may still convert, or no, to GimpRGB for now).
- app: gimp_prop_gegl_color_button_new() was added as an alternative to
  gimp_prop_color_button_new() when the property is a GeglParamColor. Eventually
  the former should replace completely the latter.
- libgimpwidgets: gimp_prop_color_area_new() now works on GeglParamColor
  properties only.
- libgimp: gimp_procedure_dialog_get_widget() will generate a GimpColorArea for
  GeglTypeParamColor arguments.

app/pdb/text-layer-cmds.c

@@ -76,20 +76,23 @@ text_layer_new_invoker (GimpProcedure         *procedure,
 
   if (success)
     {
-      GimpText *gimp_text;
-      GimpRGB   color;
+      GimpText  *gimp_text;
+      GeglColor *color;
+      GimpRGB    rgb;
 
-      gimp_context_get_foreground (context, &color);
+      color = gimp_context_get_foreground (context);
+      gegl_color_get_pixel (color, babl_format ("R'G'B'A double"), &rgb);
 
       gimp_text = g_object_new (GIMP_TYPE_TEXT,
                                 "text",           text,
                                 "font",           font,
                                 "font-size",      size,
                                 "font-size-unit", unit,
-                                "color",          &color,
+                                "color",          &rgb,
                                 NULL);
 
       layer = GIMP_TEXT_LAYER (gimp_text_layer_new (image, gimp_text));
+      g_object_unref (color);
       g_object_unref (gimp_text);
 
       if (! layer)