Get and set XDG user dirs variables in config file

Config file is $XDG_CONFIG_HOME/user-dirs.dirs.
Pass by const reference instead of value.

There is no need to have an value copy, so an const reference is faster.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Merge pull request #1 from luis-pereira/colab-xdg-user-dirs

Colaboration on xdg user dirs