[merge request] Refactoring of user.jl.

[Teika Kazura <teika lavabit com>]

Hi. Please merge my branch. It is a refactoring, and (almost) does not
affect users and API. (So Matthew doesn't have to worry any. :)

Some details:
* Desktop environment detection cleanups. Detection of say KDE is
  done by kde.jl. So far, some variables are `defvar'-ed in several
  places, but they are centralized to `user.jl'. It is now less
  bug-prone. `standalone.jl' is deleted.
* User.jl looks simpler.
* A bit of doc.

Files integration/*.jl must be correct: I ran Sawfish with fake 
environment variables, and invoked them. (xfce.jl can't be run this
way, so tried a version with (or ... t) added to the detection part.)
Correct menus appeared.

Patches look simpler with git show -b. (Ignoring whitespace diff.)

Phew,
Teika (Teika kazura)