Use XDG paths and directories for both savegame and colobot.ini.

Of course that works best under Linux but would fallback correctly on the previous behaviour (I just changed the savegame directory to say colobot-savegame).

See http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html for the full spec.

As far as I'm concerned, that was the last mandatory step before releasing a versionned tarball for wider consumption; what about pushing 0.1.0 (labelled 'alpha', of course) after that ?

Cheers, OdyX