Prevent qtconsole frontend freeze on lots of output. The output from the kernel is now clipped to last `buffer_size` before displaying and a timer is used to flush the pending output text instead of attempting to display text on every stream output from kernel. The timer interval is adjusted based on actual time taken to append a screenful of text to widget. This throttles the widget repaints and avoids choking the Qt event loop leaving time to handle other Qt events. Test cases: In [1]: for i in xrange(1000000): print i In [2]: range(100000) Without this commit the first input causes the qtconsole frontend to freeze, not responding to `Ctrl+C`.