Support deleting contacts

briar-gtk/briar_gtk/actions/window.py

briar-gtk/briar_gtk/containers/main_window.py

briar-gtk/briar_gtk/widgets/undo_notification.py

briar-gtk/data/ui/app.briar.gtk.gresource.xml

briar-gtk/data/ui/chat_menu.ui

briar-gtk/data/ui/main_window.ui

briar-gtk/po/ar.po

briar-gtk/po/briar-gtk.pot

briar-gtk/po/de.po

briar-gtk/po/es.po

briar-wrapper