If there are no messages in the queue and a user starts to
type then we can safely hide the message label since the
user has probably already read it.
This fixes a weirdness where "Incorrect Password" messages stay
around, even as the user types in the new correct password.
https://bugzilla.gnome.org/show_bug.cgi?id=704817
There's quite a bit of duplicated code between the login dialog
and the unlock dialog dealing with the various signals from the
ShellUserVerifier.
This commit moves that duplicated code into the AuthPrompt.
https://bugzilla.gnome.org/show_bug.cgi?id=704707