fix for echo_stdout_stderr attribute

[jojoyess]

• The echo_stdout_stderr attribute had no effect on the Multiline element when reroute_stdout AND reroute_stderr were True, which is the case for the default Output element.
• Everything got redirected to the Multiline element without echoing the text to the console.
• These changes do not change the behavior when only reroute_stdout OR only reroute_stderr is True.
• It now checks if the Window has the _original_stdout / _original_stderr attribute and then writes the text to it when echo_stdout_stderr is True.