Add a core example of using Tkinter with Twisted

[jesstess]

We talk about how to integrate Tkinter in  http://twistedmatrix.com/documents/current/core/howto/choosing-reactor.html#auto15, but we have no explicit, basic example in doc/core/examples.

We do already have a PyUI demo (doc/core/examples/pyuidemo.py).

[lesleylam]

Example of integrating Tkinter with Twisted

[lesleylam]

I've written up a basic example for integrating Tkinter with Twisted. Attached is the patch file for review.

[itamar]

Thanks for the contribution - looks good!

I will make three minor tweaks and then commit:

1. If possible, should avoid using "from module import *", it fills up the module with unexpected names and makes it harder to figure out where objects came from. So I'll be doing "from Tkinter import Frame, Button" etc.. Apparently some GUI frameworks recommend * imports because they have so many objects, though.

2. I'll rephrase the news file slightly.

3. The file seemed to have mixed tabs and spaces, which can be problematic.

[itamarst]

(In [34214]) Merge patch: Tkinter/Twisted integration example.

Author: lesleylam Review: itamar Fixes: #5631

[thijs]

The example wasn't included in the index.xhtml.