Capture stdout, stderr to messages textview in playpen

[gogins]

In the Csound Playpen, capture not only the C runtime library stderr (and thus Csound's default runtime messages) but also the Python stdout and stderr, and redirect their streams to the messages Gtk.TextView.

Should be possible following this: https://eli.thegreenplace.net/2015/redirecting-all-kinds-of-stdout-in-python/

[gogins]

• [x] Make a clean shutdown. This may be irrelevant as the watch on stderr continues through the life of the program.
• [x] Accumulate characters from the pipe into a string buffer and enqueue them line by line. No, use a GLib.IOChannel created from a pipe duplicated from the stderr file descriptor, and add a watch to it.
• [x] Rewrite code for greater clarity.
• [x] Ensure Csound messages don't bunch up.
• [x] Ensure that the user interface doesn't freeze up.
• [x] Configurable logging levels -- at least verbose or not.