Separate first sidebar logic into controller/view

briar-gtk/briar_gtk/controllers/__init__.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