Cooja cleanup

With the new 2.7 release out, and the ongoing cleanup of Contiki, I finally took the time to make some long overdue Cooja changes.

* Renamed Cooja's GUI class to Cooja. This is the very first file committed to Cooja, so it took more than 7 years to correct this filename. :wink:

* Renamed Java packages to match our contiki-os.org instead of non-Contiki domains.

This pull request contains no functional changes, but is an important step towards a coherent and maintainable source tree in Contiki.