Protect sensitive data from snooping over D-Bus session bus

[iinuwa]

Is using a signal for state changes safe? Signals are broadcast on the bus, which may leak data. The QR code in hybrid flow contains a shared secret, so it would be good to protect that for defense-in-depth.

Do we need to have the client set up an endpoint to send a unicast message instead?

This would be a breaking API change for initiate_event_stream().

[iinuwa]

Research: you can send signals to a specific peer; that may get around this security issue.

[iinuwa]

We are now using unicast signals. We have also implemented sending the client PIN from the backend to frontend using memfd_secret. We should do the same for the QR secret, and we can call this good.

[iinuwa]

Completed in #179 and #162.