UnlockDialog: allow typing before the first question from PAM

If the user starts typing right away, assume that the entry is
for a password and don't clear it when the secret request actually
comes. Then, if the user completes typing, we also stash the answer
and send it to GDM right away on the first PAM prompt.

https://bugzilla.gnome.org/show_bug.cgi?id=681576