GtkApplication: lock gdk on incoming messages

When we have incoming activations or action invocations we should
acquire the GDK lock, just in case the program in question is using gdk
threads.

https://bugzilla.gnome.org/show_bug.cgi?id=665737

gtk/gtkapplication.c

@@ -322,6 +322,8 @@ gtk_application_before_emit (GApplication *application,
   const gchar *key;
   GVariant *value;
 
+  gdk_threads_enter ();
+
   g_variant_iter_init (&iter, platform_data);
   while (g_variant_iter_loop (&iter, "{&sv}", &key, &value))
     {
@@ -345,6 +347,8 @@ gtk_application_after_emit (GApplication *application,
                             GVariant     *platform_data)
 {
   gdk_notify_startup_complete ();
+
+  gdk_threads_leave ();
 }
 
 static void