Code review comment for lp:~widelands-dev/widelands/add-xdg-support

I have added some comments for more code improvements. Thanks for taking this on :)

To comply with the standard, we still need another directory defined, which is $XDG_CONFIG_HOME and where "config" should sit.

At the moment, we have const std::string kConfigFile = "config"; defined in src/logic/filesystem_constants.h

The file is read/written in src/ui_fsmenu/options.cc and src/wlapplication.cc