bug#37248: GNOME and Icecat problems

[jon]

There are several problems with using GNOME 3.28 and Icecat 60 on Guix, many of 
which impact each other.

1. The gnome-tweaks package can't find gnome-shell extensions from the 
gnome-shell-extensions
package, and so claims none are installed.  
This is a packaging problem because on normal distributions, packaged 
extensions are visible to gnome-tweaks.
2. The stock "Web" browser is not able to install shell extensions.  This may 
be an upstream bug.
3. Icecat can not install gnome-shell extensions because the Native Connector 
is not packaged in
Guix.  Despite the misleading name, this is the tool needed.

    https://wiki.gnome.org/Projects/GnomeShellIntegrationForChrome/Installation

4. Icecat is unable to sign in to Firefox sync.  This is apparently fixed in 68.