greeter: track various properties persistently
There is a fundamental race condition between PamAuthenticator and the greeter views. PA is a singleton that emits signals that result in stateful changes to the views (e.g. showing the prompt message). Meanwhile views are added when they get connected. This leads to circumstances where pam may have sent a prompt before a view was created, meaning the view is in an incorrect state. To mitigate the problem we'll force-emit a number of stateful signals whenever they are connected to. Thus ensuring that greeter views definitely receive the most recent states. The more long-term solution should likely be to move to property bindings instead, so these stateful signals are now also exposed as stateful properties. BUG: 456210