Multi-module projects

This proposal is to have projects that consist of more than one module, where the main module imports others. The UI, I suppose, would look like tabs within the editor area. The goal would be to keep this as simple as possible for users who don't want it. I am not interested in making the UI more complex for beginners in order to support a few advanced students; but it would be good to have a reasonable path for those advanced students.

This also makes Haskell-mode much more appealing as a general-purpose tool for learning Haskell.