Related to #66

Followup to !63

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

This reverts commit 9bfa6155.

Closes #54

Fixes #63.

The username and time in messages will be shown if either
* the author changes
* or the time difference of two messages is greater than 60 seconds

Closes #53.

Related to #53. This already improves UX a lot, but I still think we
should omit the username if it doesn't change between messages. Thinking
about it, we could even omit the time if it doesn't change.

The color used is Briar's primary "Lime" color, as per
* https://briar-styleguide.netlify.app/brand/
* https://code.briarproject.org/briar/briar-styleguide/-/blob/2e8fa50a2628ce685ad2a3c88337388b29f496ba/layouts/partials/brand/colors.html#L15

Fixes #62.

Fixes #60. Now we need to either implement #63 or #64.

Fixes #12

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.