Closes https://code.briarproject.org/briar/briar-gtk/-/milestones/4

As of August 7, [a first beta release is available](https://mamot.fr/@KekunPlazas/104648142113069390).

The code to build can now be downloaded from [gnome.org](https://download.gnome.org/sources/libhandy/0.90/).

A [migration guide](https://gnome.pages.gitlab.gnome.org/libhandy/hdy-migrating-0-0-to-1.html) is available, just as [packages for Debian](https://packages.debian.org/bullseye/libhandy-1-0).

Fixes #59.

* Show contact's connection state in list
* Support copying content in messages (#51)
* Fix bug with Briar messages not having text (#56)
* Fix bug with contact's name not showing in private messages (#55)
* Update Briar core to version 1.2.9
* Lots of software tests

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

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

This is a squash merge of `complete-revision`.

Parts of the changes are heavily inspired by GNOME Lollypop.

Modules can't contain slashes and class names seem to follow CamelCase
style.

Based on a template generate by Gnome Builder.