First steps analogous to
https://salsa.debian.org/xmpp-team/sms4you/-/commit/6dcf2ccc13d4d2f962fafb96ba9a9ac54ce9fa18.

As mentioned correctly by @grote in
604c4e5f (comment 39593).

Closes
#2.

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 remaining from Briar GTK gets modified to
match python-briar-wrapper's needs.