Add support for Excel import

This adds support for Excel spreadsheets on both the API level as well as the GUI (import wizard). Refer to `Example2b` for details on how to use it programatically. The GUI component is enabled and can be used right away.

**Don't forget to add the POI JARs to the build path, otherwise it might not work correctly.**

There are various refinements and cleanups on top of that:

 - Refinements of (Javadoc) comments
 - Some refactoring to make interface more usable
 - A few UI bug fixes