script: Support translatable strings for properties

ClutterScript should be able to automatically call gettext() and friends
on strings loaded from a UI definition, prior to passing the string to
the object it is constructing.

The basic implementation is trivial:

  - set a translation domain on the ClutterScript instance
  - mark the translatable strings inside the JSON data, like:

      "property" : {
        "translatable" : true,
        "string" : "a translatable string"
      }

The hard part is now getting the tools we use to extract the
translatable strings to understand the JSON format we use inside
ClutterScript.

clutter/clutter-script-private.h

@@ -130,6 +130,9 @@ gboolean _clutter_script_parse_color       (ClutterScript   *script,
                                             ClutterColor    *color);
 GObject *_clutter_script_parse_alpha       (ClutterScript   *script,
                                             JsonNode        *node);
+gboolean _clutter_script_parse_translatable_string (ClutterScript *script,
+                                                    JsonNode      *node,
+                                                    char         **str);
 
 void _clutter_script_construct_object (ClutterScript *script,
                                        ObjectInfo    *oinfo);