This also adds another out parameter to pw_strength(), which is a discrete
indication of the password strength (to be used with GtkLevelBar). The
advantage of having such a parameter back from pw_strength() is that we
can make sure the fill value and the hint string change at the same
time, for better consistency.
https://bugzilla.gnome.org/show_bug.cgi?id=679984
Passing the username and the old password allows the password
quality check find more bad passwords. Also, add a way to provide
more information about why a password is not good enough.
https://bugzilla.gnome.org/show_bug.cgi?id=676396
