Abstract resources directory path in containers

briar-gtk/briar_gtk/container.py

briar-gtk/briar_gtk/containers/add_contact.py

briar-gtk/briar_gtk/containers/login.py

briar-gtk/briar_gtk/containers/main_window.py

briar-gtk/briar_gtk/containers/private_chat.py

briar-gtk/briar_gtk/containers/registration.py

briar-gtk/briar_gtk/define.py