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
```

This wasn't possible before because my version of flatpak was buggy.