user-accounts: port to libaccountsservice

This removes a bunch of duplicated code, and also drops
a direct dependency on libsystemd-login.

https://bugzilla.gnome.org/show_bug.cgi?id=671858

panels/user-accounts/um-fingerprint-dialog.h

@@ -18,11 +18,11 @@
  */
 
 #include <gtk/gtk.h>
-#include "um-user.h"
+#include <act/act.h>
 
 gboolean set_fingerprint_label (GtkWidget *label1,
                                 GtkWidget *label2);
 void fingerprint_button_clicked (GtkWindow *parent,
                                  GtkWidget *label1,
                                  GtkWidget *label2,
-                                 UmUser    *user);
+                                 ActUser   *user);