In preparation to future notification logic, the urgency hint should
already get their own logic.

Related to ff4e91e4 and #57.

Using GTK's Window.set_urgency_hint: https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/Window.html#Gtk.Window.set_urgency_hint

Additional info: https://specifications.freedesktop.org/wm-spec/wm-spec-latest.html#URGENCY

Closes #57.

Related to #27.

Related to #27.

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.

It's much easier to maintain like that.

Also add Arabic.

* 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

Some messages are notices that don't carry any text (responses to forum invitations for example).

```
Traceback (most recent call last):
  File "/usr/share/briar-gtk/briar_gtk/containers/private_chat.py", line 101, in _add_message
    message_widget = PrivateMessageWidget(self._contact_name, message)
  File "/usr/share/briar-gtk/briar_gtk/widgets/private_message.py", line 18, in __init__
    self._setup_view(contact_name, message)
  File "/usr/share/briar-gtk/briar_gtk/widgets/private_message.py", line 34, in _setup_view
    message["text"])
KeyError: 'text'
```

Fixes #56.

Fixes #55.

Fixes #51.

Using the icons from the Android client:
https://code.briarproject.org/briar/briar/-/blob/release-1.2.9/briar-android/src/main/res/drawable/contact_connected.xml

Fixes #19.

briar-headless.jar and python dependencies with flatpak-pip-generator.

Briar GTK now searches for the Briar Headless JAR at three places:
* _/app/share/java/briar-headless.jar_
* _/usr/share/java/briar-headless.jar_
* _~/.local/share/java/briar-headless.jar_

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

Too many lines missing ^^

This reverts commit 6b39784a.

Now with 96% test coverage.

Related to #2.

Related to #26

Related to #50.

Generated with the Open Age Ratings Service:
https://hughsie.github.io/oars/index.html

That jar got build from the most recent commit

briar@f61e2b39

of the merge request adding the `lastChatActivityKey`

briar!1242

This now validates with
```
appstream-util validate app.briar.gtk.xml.in
```