Support sub-presentation (project) management

As the option to create a new project to the same folder as the old one does not exist in the new version of Studio, we need to add a "Create sub-presentation" or some such to the File menu to duplicate the old behavior.

Now creating sub-presentations is a pain, as you must first create a new project somewhere, then copy the stuff from it to the one you want to use it as a sub-presentation in, and then add it as a sub-presentation using the sub-presentation dialog.

The old system did all that with one checkbox in the file dialog (uncheck the "Create directory for project").