behaviour: Implement the implicit alpha parsing

ClutterBehaviour should implement the Scriptable interface
and parse ClutterAlpha when implicitly defined, instead of
having this ad hoc code inside ClutterScriptParser itself.

After all, only ClutterBehaviour supports Alpha defined
implicitly.

clutter/clutter-script-private.h

@@ -121,8 +121,8 @@ gboolean clutter_script_parse_geometry    (ClutterScript   *script,
 gboolean clutter_script_parse_color       (ClutterScript   *script,
                                            JsonNode        *node,
                                            ClutterColor    *color);
-GObject *clutter_script_parse_alpha       (ClutterScript   *script,
-                                           JsonNode        *node);
+GObject *_clutter_script_parse_alpha       (ClutterScript   *script,
+                                            JsonNode        *node);
 
 void _clutter_script_construct_object (ClutterScript *script,
                                        ObjectInfo    *oinfo);