Closes #15.

* https://code.briarproject.org/briar/briar/-/wikis/changelog#anchor-127
* https://code.briarproject.org/briar/briar/-/tags/release-1.2.7
* https://media.dorfbrunnen.eu/briar/jars/009/

Fixes #13

Thanks to libhandy [1], this is adaptive to different screen sizes.
This commit is heavily based on the work done at GNOME Fractal [2]
and GNOME Lollypop [3]. Huge kudos to them.

[1]: https://source.puri.sm/Librem5/libhandy
[2]: https://gitlab.gnome.org/GNOME/fractal/tree/4df16251f405335d3a7007301530d158fdcf774b
[3]: https://gitlab.gnome.org/World/lollypop/tree/1.2.20

Developed by Purism at https://source.puri.sm/Librem5/libhandy

Documentation at https://honk.sigxcpu.org/projects/libhandy/doc/

Sometimes it seems to work. Only in startup classes it can't access
it...

Also fix do_shutdown function.

The quit function was actually never called before. That was the reason
why the Java background process never stopped.

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.

With links back to base projects.

Also split up login and registration.