transcriberbin: notify passthrough at the appropriate time
We want to enable passthrough internally, and only notify that internally it has been enabled once the transcriber has been unlinked.
This way applications connected to the notify handler can synchronously update the properties and attempt to disable passthrough again.
Doing so properly requires a refactoring of the transition to the passthrough state, with the currently set passthrough mode maintained separately from the target passthrough state.
Also adds an example to demonstrate the behavior