Add __init__ files for modules

Although it isn't necessary for Python 3, some tools analyzing code
(e.g. pyreverse from pylint) need those files.

briar-gtk/briar_gtk/__init__.py

+# Copyright (c) 2020 Nico Alt
+# SPDX-License-Identifier: AGPL-3.0-only
+# License-Filename: LICENSE.md
+"""
+A GTK application for Briar
+"""

briar-gtk/briar_gtk/actions/__init__.py

+# Copyright (c) 2020 Nico Alt
+# SPDX-License-Identifier: AGPL-3.0-only
+# License-Filename: LICENSE.md
+"""
+GTK Actions of Briar GTK
+"""

briar-gtk/briar_gtk/containers/__init__.py

+# Copyright (c) 2020 Nico Alt
+# SPDX-License-Identifier: AGPL-3.0-only
+# License-Filename: LICENSE.md
+"""
+Containers in Briar GTK
+"""

briar-gtk/briar_gtk/widgets/__init__.py

+# Copyright (c) 2020 Nico Alt
+# SPDX-License-Identifier: AGPL-3.0-only
+# License-Filename: LICENSE.md
+"""
+Widgets (small UI chunks) in Briar GTK
+"""