[Urwid] Twisted version of generic_main_loop

[Walter Mundt]

Ian Ward wrote:
> It's a good start.  I was hoping this could be done with
> get_input_descriptors[1] and get_input_nonblocking[2] instead of using
> threads.

Sure it can, but when I wrote this I was just barely starting on 
Twisted, so I wasn't sure where to find the API to make it happen. 
That's on my to-do list.  For now, this solution works.

-Walter