fixed possible threading problems in cob_command_gui 

The gtks emstop label/image was set within the subscribers thread, without any thread protection. Calls to gtk functions from other threads out as the main thread must be protected with gtk.threads_enter() and gtk.threads_leave().

This fix refers to ticket #446. Since it was not possible for me to reproduce the errors, I'm not sure whether the bugfix will help. This should be tested!