Avoid using gdk_window_freeze_updates() to implement Freeze()

Implement Freeze() by blocking the GtkWindow "expose-event"/"draw" signal
instead. Since the introduction of client-side windows in GTK+ 2.18,
gdk_window_freeze_updates() is unuseable because the impl_window (and thus the
update_freeze_count) for a given GdkWindow can change unpredictably. See #16795

include/wx/gtk/window.h

@@ -383,6 +383,7 @@ protected:
     virtual void DoFreeze() wxOVERRIDE;
     virtual void DoThaw() wxOVERRIDE;
 
+    void GTKConnectFreezeWidget(GtkWidget* widget);
     void GTKFreezeWidget(GtkWidget *w);
     void GTKThawWidget(GtkWidget *w);
     void GTKDisconnect(void* instance);