openbox: Add units for Openbox

These are installed conditionally, only when Openbox is present.
Units for Steam - big picture mode.

These are useful for folks making a "steambox", or just to
close all your other appps while gaming. You can of course

   systemctl --user isolate steam.target

to switch to this.

Note that you probably should still run a window manager by
linking e.g. xfwm4.service into steam.target.wants.
Set Restart=on-failure for all relevant units.

This is a decent default for all of these units.