Clean up with help of linters

briar-gtk/briar_gtk/actions/window.py

briar-gtk/briar_gtk/controllers/chat_menu.py

briar-gtk/briar_gtk/controllers/main_menu.py

briar-gtk/briar_gtk/controllers/main_window.py

briar-gtk/briar_gtk/controllers/private_chat.py

briar-gtk/briar_gtk/controllers/sidebar.py

briar-gtk/briar_gtk/views/chat_menu.py

briar-gtk/briar_gtk/views/main_menu.py

briar-gtk/briar_gtk/views/main_window.py

briar-gtk/briar_gtk/views/private_chat.py

briar-gtk/briar_gtk/views/sidebar.py

briar-gtk/briar_gtk/window.py

briar-gtk/data/ui/main_window.ui