The initial implementation was just calling signal-cli directly
using `subprocess.run`.

Going with DBus makes it easier to shield signal-cli from the
rest of the system. It also makes sure the signal-cli daemon is
running in the background and receiving messages. This is important
when a recipient does the "Reset secure connection" from the app. We
must receive their new keys, otherwise our future messages will
appear as "bad encrypted message" for them.

Fixes: #428