Focus driving in GtkCellArea now works.

- Fixed focus driving in GtkCellArea with refined apis
 - Added gtk_cell_area_activate() to be called when the area has focus
   (to activate or start editing the focused cell)
 - Added support for this in cellareascaffold
 - testcellarea now watches the "toggled" signal for a toggle renderer
   and updates the model state accordingly, this currently works with
   keyboard navigation, however focus is still not painted on cells.

gtk/gtkcellarea.c

@@ -675,6 +675,7 @@ gtk_cell_area_real_activate (GtkCellArea         *area,
 				       &background_area, flags))
 	return TRUE;
     }
+
   return FALSE;
 }