Login: sensitivity fixes

The login dialog had these issues:
  - the entry was not really disabled, you could still edit text
  - the sensitivity state was not reset on verification failure
  - the session list was not disabled

The unlock dialog had these issues:
  - "Login as another user..." was not insensitive
  - redundant password char setting, overwriting the one given by the
    question

The entry insensitive style was also wrong.

https://bugzilla.gnome.org/show_bug.cgi?id=687113