Notify the user that the scenes list is loading.

[Peregrine05]

If the scenes list is very long, then it can take much time to finish loading all of the scenes after the Load button is clicked. The worlds list also takes long to load after the Change world button is clicked; however, it notifies the user that the worlds list is being loaded with the text Loading worlds list.... The scenes list should notify the user with similar text, such as, Loading scenes list..., instead of remaining closed until the scenes list is loaded.