Hello Guix,

This new version of the patch addresses the following:
- inherits definition from icecat[1];
- use source tarballs for tor-browser and tor-launcher[2];
- does not define torbutton anymore (it is bundled with tor-browser);
- unbundles some system libs (nss, nspr, zlib, libevent and icu)

1. There are four phases which are almost exact copies from icecat.
I've tried to use let, define and package-properties to remove the
hardcoded strings which are in the way of inheritance, but I've had
no success. The phases are: 'install-icons,
'apply-guix-specific-patches, 'neutralise-store-references and
'wrap-program. Maybe someone here can shed a light.

2. Those tarballs and also the one used for fonts are not kept for
long on dist.torproject.org. Usually they are removed when there is
a new browser version. So this will be a problem for time-machine.
On the other hand, the git repos for fonts and torbrowser source
are huge, so I'm unsure which would be preferable.