feature request: improve writing to stdout/stderr from background thread

[zpincus]

In reference to IPython issue 10406, a useful prompt_toolkit enhancement would be to more gracefully deal with text sent to sys.stdout / sys.stderr from a background thread.

Per @jonathanslenders, this would entail using os.openpty() to create a new pty, then use os.dup2 and fiddle with the file descriptors to make sure that prompt_toolkit writes to the original stdout, while all stdout objects will point to a proxy object while the interface is visible. This would need a reader for the PTY master FD in the event loop.