all world's properties can now be edited
move utils to a specific package

These clases are ugly, as they implement only static methods, but they
are very useful.
use the new formalism to stringify colors in BuggleWorld I/O
buggle entities are now correctly handled on world I/O
write the buggles in the worlds on disk for the worlds we have
readFromFile() is now a static method constructing a new world

This could have been a constructor, but we already have World(name)
using this signature.

That's too bad because java does not allow to override static methods,
which feels strange: I cannot put the @Override annotation...