Add profile and savegame fetchers in SystemUtils

This breaks the tests compilation. :/
Add Linux-specific savegame and profile settings according to the XDG Base Directory Specification

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
BROWN-PAPER COMMIT: Drop the tests to have the thing build

Someone understanding GMock / GTest needs to replace that commit (and the two parents) with a working setup
Merge pull request #151 from OdyX/dev-use-xdg-homedirs

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