Fixes #16

Related to:
* briar-gtk#19
* briar!1260

briar-debian!1

Seems like older versions lack part of the api we're using in
briar_wrapper.

Linux Mint 19.1 Tessa is still on websockets version 3.4, so we should
really require a more recent version here.

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

In case something fails due to this, it's hard to debug it without
knowing the exact auth_token. Fuzzy testing should be done at another
place.

Related to
briar-gtk#44 and
briar-gtk#45.

This commit makes signal ids stable by always incrementing the id
number.

Similar to GTK's signals, listeners for events can now be added to
SocketListener.

Set up flit for briar-wrapper

See merge request !2

Correct faq entry on contact transfer

See merge request !3

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

Closes
#2.

Unit test shouldn't fail in GitLab CI

See merge request !1

In Briar GTK, this stage is failing for a reason I don't understand:
briar-gtk#2

Here, it seems to work.

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.

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.

If our alias matches the contact's nickname, it seems like Briar only
exposes the author's nickname and leaves out the alias field.

Also refactor socket_listener.py to make it more general.