gdm: add fingerprint support

This commit adds the ability to log in with a fingerprint instead
of a password (assuming the user is enrolled and fingerprint
isn't disabled via gsettings)

https://bugzilla.gnome.org/show_bug.cgi?id=657823

data/theme/gdm.css

@@ -30,6 +30,10 @@
     min-width: 350px;
 }
 
+.login-dialog-prompt-fingerprint-message {
+    font-size: 10.5pt;
+}
+
 .login-dialog-user-list-view {
     -st-vfade-offset: 1em;
 }