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

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.

Start by tracing prompt_toolkit's handling of stdout, stderr, and the event loop; no specific files or tests are named. Review how os.openpty and os.dup2 could support a proxy for stdout objects while the interface is visible, with a reader for the PTY master file descriptor. Done means background-thread output is handled gracefully while prompt_toolkit continues writing to the original stdout.