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

* 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

Analogous to:
* https://salsa.debian.org/xmpp-team/sms4you/-/commit/6dcf2ccc13d4d2f962fafb96ba9a9ac54ce9fa18.
* https://salsa.debian.org/jbicha/simple-scan