Some subscribers are not disposed

@Feichtmeier I did a simple test:

I added a print in the subscriber callback of the displays model ;
as expected, it prints a message on loading the display page, and also when I update the screen config in GCC ;
but if I leave the page, the subscriber is still active and prints each display config change ;
more problematic, each time I reload the page, it adds a subscriber, which mean a change will be notified multiple times and with multiple same print.

So I suppose this subscriber should be cancelled:

I also noticed that the bluetooth device subscriber is also left uncancelled (with the same behavior):

ubuntu-flutter-community / settings