Implement gdk_screen_get_primary_monitor in all GDK backends

The implementations are all trivial, just returning 0. At least
for Quartz, we could probably do better than that. Bug 604459.

gdk/win32/gdkscreen-win32.c

@@ -66,6 +66,14 @@ gdk_screen_get_n_monitors (GdkScreen *screen)
   return _gdk_num_monitors;
 }
 
+gint
+gdk_screen_get_primary_monitor (GdkScreen *screen)
+{
+  g_return_val_if_fail (screen == _gdk_screen, 0);
+
+  return 0;
+}
+
 gint
 gdk_screen_get_monitor_width_mm (GdkScreen *screen,
                                  gint       num_monitor)