Switch back to Xfce

I liked i3, but in the end tiling window managers don't fit
well with the way I use my computer.

* pulseaudio-oneshot: add back pnmixer
* compton: remove shadows (vsync is all we need)
* dmenu-session: spotted a bug where logging out of i3 does not work
(see i3/autostart)
* xinitrc: exec startxfce4