Add default values for `$XDG_DOWNLOAD_DIR` and such

[netzego]

Correct me if i am wrong. The standard for an unset/not exported variable eg. $XDG_DOWNLOAD_DIR should default to $HOME. See: https://www.freedesktop.org/wiki/Software/xdg-user-dirs/.

Here is a shellscript example of how to find the desktop and the download directory:
test -f ${XDG_CONFIG_HOME:-~/.config}/user-dirs.dirs && source ${XDG_CONFIG_HOME:-~/.config}/user-dirs.dirs
echo ${XDG_DESKTOP_DIR:-$HOME/Desktop}
echo ${XDG_DOWNLOAD_DIR:-$HOME}

I suggest to implement this behavior as well.