Add copyright notices to all files

At Briar GTK I respected https://reuse.software/, with file headers looking like this:

# Copyright (c) 2019 Nico Alt
# Copyright (c) 2014-2018 Cedric Bellegarde <cedric.bellegarde@adishatz.org>
# SPDX-License-Identifier: AGPL-3.0-only
# License-Filename: LICENSE.md
#
# Initial version based on GNOME Lollypop
# https://gitlab.gnome.org/World/lollypop/blob/1.0.2/lollypop/define.py

https://code.briarproject.org/briar/briar-gtk/-/blob/0.1.0-beta3/briar-gtk/briar_gtk/define.py

For our project I would suggest something like this:

/*
 * SPDX-FileCopyrightText: 2021 The Briar Project <contact@briarproject.org>
 * SPDX-License-Identifier: AGPL-3.0-only
 * License-Filename: LICENSE.md
 */