Convert `get_protocol_state` to an automatic property, so that rather
than requiring explcit signals for lifecycle events, a GObject notify
signal can be used instead.
Convert disconnect signal to a property so it can be accessed if needed.
Convert code listening to the disconnect signal to listen to notify
signals for `protocol_state` instead, and hence also treat the session
as disconnected when a logout is in progress.
Fixes#986
41be8693d4