Openbox: Add obconf.

[ng0]

[PATCH] gnu: Add obconf.

I noticed we don't have obconf, the configuration tool for openbox.
Nix creates an wrapper for this application 
(https://github.com/NixOS/nixpkgs/blob/56904d7c423f2b13b37fbd29f39bbb4b52bc7824/pkgs/tools/X11/obconf/default.nix#L18)
 I don't know if we'll need this. Running from within openbox it seemed to 
execute and go through the tabs without crashing.
I haven't tried changing any settings so far.