Show contacts and chat history of first contact

src/briar/api/models/contacts.py

src/briar/api/models/model.py

src/briar/api/models/private_chat.py

src/briar/gtk/containers/chat.py

src/briar/gtk/containers/main.py

src/briar/gtk/containers/startup.py

src/briar/gtk/window.py