Introduce a state-changed signal which also gives a reason for why the state
changed. This will allow the UI to give some meaningful feedback to the user.
Additionally we can get rid of a number of things that were not really states,
but rather reasons for why a state changed (f.e. authentication failures).
This interface should be implemented by origins which represent an online
account. This means they must have a implement a property representing the
credentials and another representing the account state (online, offline, etc).
