Implementing UI to enable/disable backends

As discussed last year, adding a UI to disable/enable the backends on-the-fly. For my use-cases the Lan backend will never work (due to VPNs, WiFi client isolation and my phone not necessarily being on the same WiFi connection as my PC); so I'd like to be able to disable it completely.

Additional benefit: if we ended up with a situation where the Bluetooth backend was causing chaos for people (I've seen a few bugs with it being implicated in crashes etc) then it will provide the user with the ability to disable it if they don't need it without recompiling.