bug#26454: [PATCH 2/2] gnu: add emacs-exwm

[Arun Isaac <arunisaac@systemreboot.net>]

I have finished all corrections. And, as far as I can tell, exwm builds
and works fine. If there are no other suggestions, let me know, and I'll
push.

In the patch I just sent, I check if the 'exwm feature is loaded. If it
is not loaded, then a default configuration is loaded. Else, nothing
happens and we proceed assuming the user has configured exwm in their
~/.emacs. This way, we get a working exwm out of the box and at the same
time, retain the user's ability to customize their exwm easily.

Though upstream prefers we use the pregenerated source code, I'm afraid
we cannot do that. All Guix packages are strictly built from source. For
now, there is no problem with our own regenerated source code. And, I
don't anticipate there will be many problems in the future. If and when
there is a problem and the package breaks, we will report the problem
upstream as a bug, and take any necessary action to fix our
package. Until then, this should be fine.

I've made many other minor edits and a little code restructuring. Please
go through the patch. I had to move the synopsis field up to the top of
the package definition so that I could reference it later while creating
the .desktop file.

Also, I've switched back to the ELPA tarball.

There seem to be some exwm upstream bugs with floating windows. Apart
from that, it works great. I have switched to exwm from my previous
window manager, dwm. :-)