This partially reverts 639dff3a7c. When loading a session,
the monitor-bus that was saved with the session is used.
This changes semantics of the monitor-section/config.
Config::set_use_monitor_bus(bool) is used to initiate a change!
Notification about the change is sent asynchronously by
Session::MonitorBusAddedOrRemoved
It is no longer possible to directly call add/remove_monitor_section()
and leave the session + config in an inconsistent state.
1854a72b80