app: make "gegl:matting-levin" default engine of Foreground Select tool.

Even though this engine is optional, we already have the code to detect
its absence at runtime, and to fallback to "gegl:matting-global". So it
won't be a problem even then.

When the operation is present though, it definitely makes a lot more
sense than matting global as default, because it performs a lot better
in most cases (as far as I could see as well as others).

(cherry picked from commit b52cdf4c90)

app/tools/gimpforegroundselectoptions.c

@@ -119,7 +119,7 @@ gimp_foreground_select_options_class_init (GimpForegroundSelectOptionsClass *kla
                          _("Engine"),
                          _("Matting engine to use"),
                          GIMP_TYPE_MATTING_ENGINE,
-                         GIMP_MATTING_ENGINE_GLOBAL,
+                         GIMP_MATTING_ENGINE_LEVIN,
                          GIMP_PARAM_STATIC_STRINGS);
 
   GIMP_CONFIG_PROP_INT  (object_class, PROP_LEVELS,