extension database: emit databaseChange event after initialization.

vispa/static/js/extensions.js

@@ -901,6 +901,7 @@ define(["jquery", "emitter", "vispa/module", "vispa/ui"], function($, Emitter, M
           self.logger.error("Loading extension database failed");
         } else {
           self._database = database;
+          self.emit("databaseChange", self._database);
         }
       });
     },