Add support for XDG Path property

This only works with fluxbox, as to the best of my knowledge windowmaker
and icewm don't allow switching to another directory with cd and then
executing the command.

Thanks to Gregor Bollerhey (gregor-b on github) for making the initial
pull request about this issue.
Use standard terminal on debian-like distributions

On debian and debian-like distributions there is an x-terminal-emulator
symlink that points to the preferred terminal emulator to use. So, if
we're on debian, use this instead of defaulting to xterm.