model: improve logging messages

correctly report the number of listeners. It would report 1 listerner too much if an object is subscribing twice
model: ensure VAs are also unsubscribed when proxy goes away
gui: turn on and off  SEM stream on start/exit

needed to properly unsubscribe from the stream, so that the backend stops scanning when the GUI is closed

also clean up a bit the stream controller
gui: remove unused functions