gdm: generalize fingerprint message into login hint

gdm: generalize fingerprint message into login hint

The fingerprint message is useful for users that click their
names in the user list to let them know if fingerprint login
is available.

This same place on screen (below the login entry) can potentially
be used for other messages as well.

This commit changes the variable and style names surrounding
this feature to be more generic.

A subsequent commit will leverage this functionality to provide
a hint on how to log in to the local enterprise domain controller
(if relevant).

https://bugzilla.gnome.org/show_bug.cgi?id=681975