Make it possible to run multiple versions of briar-desktop concurrently

For interactive testing it is extremely useful to be able to start multiple UIs simultaneously and interact with them. A separate data directory can be specified already, the only thing preventing this is the hardcoded tor port (no two apps can listen on the same port). Let's see if we can make the Tor port configurable in upstream briar. I already solved this for briar-swing here. It could be done that way, or a bit differently, but I think making the tor port configurable is something useful.