Link Whisperfish as secondary device

Result of a long weekend repeating what @ntninja tried to do in !77 (closed) . I took some code from there for the registration, but didn't try any big restructuring of rust-qml interactions. Registration/linking works. However, if the Signal app is on the same phone as WF, one has to transfer a screenshot of the QR-code to an other device for scanning. Sending and receiving works fine as well, with the known quirks and maybe some more. There is no contact synchronisation, so a new chat might/will at first show up as uuid-only, until the phone number is discovered from some incoming message. The chat obviously has to be initiated from outside WF.

Implementing contact sync probably makes most sense in combination with (or after) other profile related things. I'll probably won't have time for that in the near future, so I'm just submitting this as is.