Related to
briar-gtk!96
and
briar-gtk#88.

Because the two independent Briar projects briar_repl and Briar GTK
combine forces, the api wrapper part that used to be part of Briar GTK
now becomes a new project on its own: python-briar-wrapper.

With this commit, everything specific to GTK and GNOME gets deleted.

https://gitlab.gnome.org/World/podcasts/blob/bcd610e2e377d70ff75f52bbfae4d2007a248a90/CONTRIBUTING.md
https://gitlab.gnome.org/World/podcasts/blob/master/code-of-conduct.md