Allow pairing with existing Signal app, Whisperfish as secondary device

While this app is already quite useful as it is, it won't realistically be anywhere near “completed” any time soon, if ever. As such, the most featureful Signal mobile app for SailfishOS currently is (and likely always will be) the Signal for Android app used through the compatibility layer. However, due to the way the Signal service distinguishes between master/slave devices, there can only be one master device that all other devices have to be linked with and since the Signal for Android app only supports running in master mode it becomes effectively impossible to use both apps at the same time on the same number.

As a pragmatic solution to the above, I'd like to see Whisperfish additionally supporting slave mode where it can be linked up with the Signal for Android app installed on the same, or possibly also a different, Android device. To my knowledge to Signal protocol is the same in both modes, except for the different sign-up procedure of course, but definitely proof me wrong if this is not the case.

In a lot of ways, this request can be seen as the inverse of what is requested in #97 (closed).

(From the UI side, a stupid issue here is of course that the Signal for Android app insists on taking a camera picture of the registration QR code from the slave device to initiate pairing, but this could be worked around to a certain degree by adding the ability to “share” the rendered QR code picture using the system share functionality, so that it can be send to other devices such as another mobile device, desktops, printers, etc.)